BIOLOGY 

LIBRARY 


TEXT   BOOK 


OF 


VERTEBRATE  ZOOLOGY 


BY 


J.  S.  KINGSLEY 

PROFESSOR  OF  ZOOLOGY  IN  TUFTS  COLLEGE 


HENRY     HOLT     AND     COMPANY 
1899 


Q  L  (s>  C 


BIOLOGY 

LIBRARY 

G 


GENERAL 

COPYRIGHT,  1899, 

BY 
HENRY  HOLT  &  CO. 


C.   J.    PETERS   &   SON,    TYPOGRAPHERS, 
BOSTON. 


PREFACE 


WITHIN  recent  years  the  laboratory  method  has  become  the 
basis  of  instruction  in  every  science.  The  student  is  expected 
to  find  out  a  certain  number  of  fundamental  facts  directly  from 
nature,  but  while  this  has  in  itself  great  value  as  a  training  in 
observation,  the  fullest  benefit  of  the  study  is  not  obtained 
unless  there  be  a  comprehension  of  the  bearings  of  the  facts 
observed.  Observation  and  uncorrellated  facts  do  not  make  a 
science.  Attention  can  be  directed  to  the  relations  and  signifi- 
cance of  the  facts  ascertained  in  the  laboratory  by  means  of 
lectures,  but  a  somewhat  extended  experience  has  shown  that 
the  average  student  needs  something  more  than  his  lecture 
notes,  at  least  when  beginning  any  subject.  The  present  volume 
is  intended  to  supplement  both  lectures  and  laboratory  work, 
and  to  place  in  concise  form  the  more  important  facts  and  gen- 
eralizations concerning  the  vertebrates.  It  is  also  hoped  that 
it  may  have  some  value  for  students  of  medicine  in  explaining 
many  peculiarities  of  the  structure  of  man  which  seem  mean- 
ingless unless  viewed  in  the  light  of  comparative  morphology. 
When  once  their  meaning  is  comprehended  it  is  easy  to  remem- 
ber them. 

The  first  part  of  the  volume  is  devoted  to  an  outline  of  the 
morphology  of  vertebrates  based  upon  embryology.  This  treat- 
ment has  been  adopted,  since  the  author  believes  that  in  this 
way  the  bearings  of  the  facts  can  be  most  clearly  shown  and 
most  easily  remembered.  The  remainder  of  the  volume  pre- 
sents an  outline  of  the  classification  of  vertebrates,  a  subject 
which,  in  recent  years,  has  been  too  much  ignored  in  college 
work.  Here  the  fossils  are  included  as  well  as  the  recent 
forms,  since  the  existing  fauna  must  be  studied  in  the  light  of 
the  past.  Numerous  generic  names  have  been  mentioned  with- 
out characterization  ;  they  have  been  inserted  in  order  that  the 
student  may  be  able  to  ascertain  the  relationships  of  the  forms 
he  may  find  mentioned  in  collateral  reading. 

iii 


IV  PREFACE. 

In  this  second  part  the  author  has  ventured  to  differ  in 
some  points  from  the  majority  of  American  students.  Thus 
he  has  been  unable  to  recognise  in  the  so  called  orders  of 
ornithologists  groups  of  birds  of  more  than  family  rank,  while 
their  families  are  equivalent  to  genera  in  the  other  classes  of 
vertebrates.  Again  in  the  matter  of  nomenclature  well-known 
generic  names  have  been  retained,  in  spite  of  the  law  of  priority. 
These  are  the  names  of  morphological  literature,  and  to  have 
used  Tri turns,  Molge,  Myctophium,  Zaglossus ;  to  have  mixed 
up  Esox  and  Belone  would  have  served  no  useful  end. 

A  fair  proportion  of  the  illustrations  are  original ;  as  many 
more  have  been  engraved  for  the  volume.  These  latter  as  well 
as  those  borrowed  have  been  credited  as  far  as  possible,  to  the 
original  source.  The  author  would  here  return  his  sincere 
thanks  to  Professor  Robert  Wiedersheim,  Professor  A.  S.  Pack- 
ard, and  Dr.  Bashford  Dean  for  cliches  from  their  works.  He 
would  also  acknowledge  his  indebtedness  to  Professor  C.  S. 
Minot,  Dr.  G.  H.  Parker,  and  Mr.  F.  A.  Lucas  for  assistance  in 
connection  with  the  manuscript.  While  many  hundreds  of 
special  articles  have  been  read  in  the  preparation  of  the  work, 
acknowledgement  must  be  made  to  the  aid  received  from  Wie- 
dersheim's  Anatomy,  the  Embryologies  of  Minot  and  Hertwig, 
Zittel's  Paleontologie,  Jordan  and  Evermann's  Fishes,  Wood- 
ward's Fossil  Fishes,  and  Flower  and  Lyddeker's  Mammals. 
Woodward's  Vertebrate  Paleontology  appeared  in  time  to  be  of 
assistance  in  the  correction  of  the  proofs. 

A  work  of  this  character  must  be  largely  a  compilation.  It 
is  impossible  to  settle  all  disputed  questions  by  personal  inves- 
tigation, and  one  can  only  take  those  statements  which  seem 
the  most  reasonable,  and  which  appear  to  have  the  most  sup- 
port. That  the  volume  will  be  found  free  from  error  is  more 
than  can  be  hoped.  The  only  apology  the  author  can  offer  for 
mistakes  of  judgment  or  of  fact  is  based  upon  the  large  field, 
the  enormous  literature,  and  the  conflicting  statements  upon 
many  points. 

TUFTS  COLLEGE,  May  14,  1899. 


CONTENTS. 


PART    I.  -MORPHOLOGY    OF    VERTEBRATES. 

PAGE 

Introduction       .          •         •         •         •.  • 

Introductory  Embryology  .         •         •         « 

Histology  •         •         •         •       .*•'•*.      "  •    ..  *         ':••'      *        ' 
Morphology  of  the  Organs  of  Vertebrates        .         .         -s    • 

Entodermal  Organs  .  .         ..        -         •  - 

Mouth           .  .  .,       •                  •  .-..••• 

Teeth  .         .  .  .        .        .        .  *        •      --        •         •         '9 

Tongue         .  .  ..... 

Oral  Glands  .  .         ...  »         •         •         •         °         2I 

Gills     .....  -       .        .  .....         22 

Air  bladder  .  .         .         •         •  •         •         •        "  .      '         2£ 

20 


Thyroid  Gland    ....••• 

Thymus  Gland    .         .         .         •         ......         33 

Digestive  Tract  .         .         .         •         ......         34 

40 
Liver    ........  ^ 

Pancreas      .         .         .         •         **•*___" 
Ectodermal  Structures        ........ 

Central  Nervous  System      ........ 

Spinal  Cord  .......... 

Spinal  Nerves    .         .....         • 

Brain     ....         .....         ; 

Cranial  Nerves  ......... 

Sense  Organs       ..»•••• 

Lateral  Line  Organs    ...••••  ^ 

Sense  Corpuscles  ..•••* 

Auditory  Organs  ........ 

Olfactory  Organs  .......  '^ 

Visual  Organs       ...••••*  ' 

Epiphysial  Organs         ........ 

Epidermal  and  Dermal  Structures      ......         Jj 

Skin       .         .         .         .         •         ...... 

Exoskeleton  ....•••*' 

Scales     ..........         92 

Feathers          ......... 

Hair        ......         .... 

Mesothelial  Structures        .....         • 


v 


VI  CONTENTS. 

PAGE 

Mesenteries 103 

Splanchnocoele    ..........       106 

Muscular  System         .         .         .         .         .         .         .         .         .       107 

Electrical  Organs  .         .         .         .         .         .         .         .115 

Urogenital  Organs       .          .         .         .         .         .         .         .         .116 

Mesenchymatous  Structures       ........        132 

Skeleton       ...........       133 

Vertebral  Column         ........        134 

Ribs      ...........       143 

Sternum        ..........       147 

Skull     ...........       150 

Appendicular  Skeleton          .......        167 

Organs  of  Circulation          .         .         .         .         .         .         .         .178 

Heart 184 

Aortic  Arches        .         .         .         .         .         .         -.         .          .        185 

i     Arteries 188 

\    Veins .         .         .192 

Lymph  System     .          .         .         .         .         .         .         .         .        198 

The  Segmentation  of  the  Head          .......       201 

The  Early  History  of  the  Ovum         .......       205 

The  Origin  of  the  Vertebrates 215 

PART    II.  —  CLASSIFICATION    OF    VERTEBRATES. 

Sub-phylum  Vertebrata    .         ...         .          .         .  .         .         .218 

Series  I.     Cyclostomata    .         .         .         .         .         ,         .         .         .219 

Class  I.     Marsipobranchii  .         .          .         .         .         .         .219 

Sub-Class  I.     Petromyzontes        .....       223 

Sub-Class  II.     Myxinoidei   .  .         .         .         .       224 

Ostracodermi ...       224 

Order  I.     Heterostraci  .         .         .         .         .         .224 

Order  II.     Aspidocephali 225 

Order  III.     Antiarcha    .         .         .         .         .         .225 

Series  II.     Gnathostomata .         .       225 

Grade  I.     Ichthyopsida      .  ...       226 

Class  I.     Pisces  .........       227 

Sub-Class  I.     Elasmobranchii      .....       232 

Order  I.     Cladoselachii  .....       237 

Order  II.     Ichthyotomi  .....        237 

Order  III.     Selachii 238 

Order  IV.     Holocephali 240 

Sub-Class  II.     Teleostomi 242 

Legion  I.     Ganoidea          ......       248 

Order  I.     Crossopterygii  .  249 

Order  II.     Chondrostei 250 

Order  III.     Holostei       .         .         o         .         .         .251 
Legion  II.     Teleostei         ......       252 

Order  I.     Ostariophysi 254 


CONTENTS.  vii 

FAGK 

Order  II.  Physostomi  .  255 

Order  III.  Synentognathi  .....  257 

Order  IV.  Hemibranchii  .  257 

Order  V.  Lophobranchii  .  .  .  .  .  258 

Order  VI.  Acanthopterygii  ....  258 

Order  VII.  Pediculata 266 

Order  VIII.  Plectognathi 266 

Sub-Class  III.  Dipnoi  .  .  '  <  .  .267 

Order  I.  Arthrodira .271 

Order  II.     Sirenoidea    ......  272 

Class  II.  Amphibia 274 

Sub-Class  I.     Stegocephali  ......  283 

Order  I.     Lepospondyli          .         .         .         .         .  283 

Order  II.     Temnospondyli    .....  283 

Order  III.  Stereospondyli 284 

Sub-Class  II.  Urodela  .  .  .  .  .-  ~  .  284 

Order  I.  Perennibranchiata  ....  284 

Order  II.  Derotremata 285 

Order  III.  Salamandrina  .....  285 

Sub-Class  III.  Anura 286 

Order  I.     Aglossa           ......  286 

Order  II.  Arcifera  ...'...  286 

Order  III.  Firmisternia  .  .  .  .  -  .  287 

Sub-Class  IV.  Gymnophiona 287 

Grade  II.     Amniota    .         .         .         ...         .         .         .  288 

Class  I.  Sauropsida 291 

Sub-Class  I.  Reptilia  .  .  .  .  .292 

Order  I.  Theromorpha  ..  •"""""^?  •  •  3°4 

Order  II.  Plesiosauria  .  .— •*» .  >  .  306 

Order  III.  Chelonia 307 

Order  IV.  Ichthyosauria 312 

Order  V.     Rhynchocephalia  .         .         .         -313 

Order  VI.     Dinosauria  .         .         .         .         ."  314 

Order  VII.      Squamata  .         .         .         .         .         .  317 

Order  VIII.     Crocodilia         ...         .         .326 

Order  IX.  Pterodactylia 329 

Sub-Class  II.     Aves      .         .         ...         .         .  330 

Order  I.  Saurura3  .  .  .  .  .  -  343 

Order  II.  Odontormae 344 

Order  III.     Odontoholca?       .         .         .         .         -344 

Order  IV.     Eurhipidurse         .....  345 

Class  II.     Mammalia 352 

Sub-Class  I.  Prototheria 376 

Order  I.  Monotremata 376 

Order  II.  Protodonta  ...  f  .  .  .  377 

Order  III.  Multituberculata  .  .  .  .  377 

Sub-Class  II.  Eutheria 378 

Legion  I.  Didelphia  .  .  .  ...  .  378 


Vlll  CONTENTS. 


PAGE 

Order  I.     Marsupialia    .         .         .         .         .         .  378 

Legion  II.     Monodephia  ......  381 

Order  I.     Edentata         .         .         .         .         .         .381 

Order  II.     Insectivora 384 

Order  III.     Chiroptera           .....  386 

Order  IV.     Rodentia      ......  388 

Order  V.     Ungulata       ......  391 

Order  VI.     Sirenia         ......  403 

Order  VII.     Cetacea      ......  405 

Order  VIII.     Carnivora 410 

Order  IX.     Primates      .  414 


TEXT  BOOK  OF  VERTEBRATE  ZOOLOGY. 


PART    I. 
MORPHOLOGY    OF   VERTEBRATES. 


INTRODUCTION. 

divide  all  animals  into  two  great  groups,  — the 
Protozoa,  in  which  all  the  functions  of  life  are  performed  by  a 
single  cell  which  constitutes  the  whole  animal,  and  the  Metazoa, 
in  which  the  body  is  composed  of  many  cells,  and  these  cells 
are  arranged  into  layers  and  organs  with  a  corresponding  differ- 
entiation of  functions  between  the  many-celled  organs. 

The  metazoa  in  turn  are  subdivided  into  several  groups  or 
phyla,  the  highest  of  which  is  called  Chordata,  while  the  others 
are  frequently  spoken  of  collectively  as  Invertebrata.  The 
phylum  chordata  is  characterized  by  the  possession  of  at  least 
three  features  which  occur  in  no  invertebrate,  —  a  skeletal  axis 
or  notochord  arising  from  the  inner  germ-layer  or  entoderm  ;  the 
possession  of  paired  gill  slits  connecting  the  anterior  part  of 
the  alimentary  canal  with  the  exterior  ;  and  a  central  nervous 
system  which  is  entirely  on  one  side  of  the  alimentary  canal. 
Details  concerning  each  of  these  features  will  be  given  on  sub- 
sequent pages. 

The  chordata  embrace  at  least  three  subphyla,  —  theJJro- 
chordia  or  Tunicata,  the  Cephalochordia  or  Leptocardii,  and 
the  Vertebrata,  the  subject  of  the  present  book.  It  is  possible 
that  a  fourth  phylum,  the  Hemichordia  or  Enteropneusti,  is  to 
be  included  here,  but  as  yet  there  is  not  agreement  upon  this 
point. 


INTRODUCTION. 


The  tunicates   include  a  large  number  of  marine  animals 
which  show  their  chordate  features  most  plainly  in  the  young, 


FlG.  I.  Diagram  of  a  larval  tunicate,  after  Seeliger.  A,  atrial  opening  ;  T, 
notochord  ;  E,  endostyle;  G,  gill  slits  ;  H,  heart  ;  M,  mouth  ;  N,  nervous  system  ; 
S,  adhesive  disks  ;  SV,  sensory  vesicle. 

the  adults   being  remarkably  degenerate.     These  young  have 
tadpole-shaped  bodies,  with  a  central  nervous  system  dorsal  in 

position,  a  notochord  which 
occurs  only  in  the  caudal 
region,  while  the  gill  slits 
occur  on  the  side  of  the 
pharyngeal  region.  In  the 
course  of  development  in  all 
except  the  Copelatae  (Ap- 
pendicularia,  etc.),  the  tail 
becomes  absorbed,  the  noto- 
chord being  lost,  while  the 
body  becomes  so  twisted 
that  both  gill  slits  and  vent 
empty  into  a  common  atrial 
chamber.  The  body  is  usu- 
ally fixed,  and  is  covered  by 
an  outer  coat  or  tunic. 

MM0 

The  Cephalochordia  are 
represented   by  Amphioxus> 
and  one  or  two  other  allied 
genera  which  are  decidedly 
fish-like  in  their  general  ap- 
pearance.    The  body  is  distinctly  segmented  ;  the  gill  slits  are 
very  numerous,  extending  back  along  the  alimentary  canal  to 


FIG.  2.  Diagrammatic  section  of  adult 
tunicate,  a,  atrial  opening  ;  />,  branchial 
chamber ;  h,  heart ;  i,  intestine;  m,  mouth; 
n,  nerve  centre ;  r,  reproductive  organ 
and  duct ;  t,  tunic  ;  vt  vent. 


INTROD  UCTION. 


the  liver  ducts,  the  stomach  thus  being  entirely  absent.  The 
notochord  extends  along  the  whole  length  of  the  animal.  These 
forms,  however,  differ  from  the  vertebrates  in  the  absence  of 
vertebrae,  in  the  peculiarities  of  the  central  nervous  system 


FIG.  3.  Diagram  of  Amphioxus,  chiefly  after  Boveri.  A,  atrium  ;  AP,  atrio- 
pore  ;  B,  branchial  clefts  ;  G,  gonads  ;  Z,  liver  ;  M,  mouth  surrounded  by  cirri ; 
A/Y,  myotomes  ;  N,  nephridia  ;  NCy  notochord  ;  S,  spinal  cord  ;  V,  vent. 

and  the  nerves  which  arise  from  it,  in  the  total  absence  of  a 
heart,  of  paired  eyes,  etc.,  as  well  as  in  the  relations  of  excre- 
tory organs,  etc.  The  species  are  few  in  number,  and  are  all 
marine,  being  found  in  the  warmer  seas  of  all  parts  of  the 
world  ;  on  our  coasts  as  far  north  as  the  mouth  of  the  Chesa- 
peake. 

The  Enteropneusti  are  decidedly  worm-like  in  appearance, 
and  their  pertinence  to  the  chordate  phylum  is  denied  by 
many.  The  so-called  notochord  is  but  a  small  diverticulum 
from  the  alimentary  tract  without  skeletal  character,  while  it  is 
not  found  that  the  segmentation  of  the  body  is  the  same  as  that 
in  the  other  chordates.  The  best-known  form — Balanoglossus 
—  was  long  considered  a  worm.  It  lives  in  the  sand  of  the 
seashore  in  many  parts  of  the  world.  Other  allies  are  so  dif- 
ferent in  appearance  {Rhabdopleura^  Cephalodiscus)  that  they 
were  long  regarded  as  Polyzoa. 

For  further  details  concerning  these  forms  reference  must 
be  made  to  works  upon  invertebrates,  and  to  the  special  papers 
dealing  with  them.  With  this  brief  reference  they  must  be 
dismissed  here  ;  for  the  purpose  of  the  present  work  is  to  deal 
with  the  single  subphylum,  Vertebrata. 


In  the  second  or  systematic  portion  of  this  volume,  the  dif- 
ferent subdivisions  of  the  group  of  vertebrates  will  be  defined  ; 


4  INTRODUCTION. 

but  as  it  is  necessary  to  use  the  names  of  several  of  the  major 
divisions  in  the  general  account  of  the  vertebrates,  a  tabular 
statement  of  classification  with  familiar  examples  is  given  here. 
Details  can  be  found  by  reference  to  the  index. 

VERTEBRATA. 

Cyclostomata  (without  true  jaws). 

Myxinoidei  (hag-fishes,  borers,  —  Myxine). 
Petromyzontes  (lampreys).  - 
Gnathostomata  (with  jaws),      a* 

Ichthyopsida  (with  gills  in  adult  or  young).    — 
Pisces  (with  paired  fins). 

Elasmobranchii  (sharks  and  skates). 

Holocephali  (elephant-fish,  —  Chimczra). 
Ganoidea  (sturgeon,  garpike,  etc.). 
Dipnoi  (lung-fishes). 
Teleostei  (ordinary  bony  fishes). 
Amphibia  (frogs,  toads,  salamanders,  etc.).   - 
Sauropsida. 

Reptilia  (lizards,  snakes,  turtles,  alligators). 
Aves  (birds).  \ 
Mammalia  (rats,  cats,  elephants,  whales,  man,  etc.). 


MBRYOLOGY. 


INTRODUCTORY  EMBRYOLOGY. 


IN  order  to  understand  clearly  the  structure  of  a  vertebrate, 
it  is  well  to  begin  with  a  short  account  of  some  of  the  phe- 
nomena of  development,  since  a  knowledge  of  the  history  of 
the  parts  will  make  their  relations,  one  to  another,  more  com- 
prehensible. The  following  outline  is  given  in  the  briefest 
manner  and  in  the  most  generalized  form,  the  various  modifica- 
tions which  are  found  in  the  different  vertebrate  groups  being 
ignored. 

All  vertebrates  reproduce  by  means  of  eggs.  These  eggs 
are  specialized  cells,  produced  by  the  female,  which  have  the 
capacity,  after  impregnation,  of  developing  into  an  animal  like 
that  which  produced  them.  The  impregnation  consists  in  the 
union  with  the  egg  of  a  still  more  specialized  reproductive 
cell,  the  spermatozoan,  produced  by  the  male ;  and  it  is  only 
after  this  union  (called  also  fertilization)  that  development  is 
possible. 

The  fertilized  egg  divides  (seg- 
ments) again  and  again,  the  result 
being  that  the  egg  is  converted  into 
a  many-celled  embryo.  At  first  the 
cells  of  this  embryo  are  arranged 
in  a  single  layer,  surrounding  a  cen- 
tral segmentation  cavity  (Fig.  4). 
Next,  those  cells  upon  one  side  of 
the  embryo  become  pushed  inside 
of  the  others  (invaginated)  much  in 
the  same  way  that  one  might  push 
in  one  side  of  a  hollow  rubber  ball, 
the  result  being  partially  to  oblit- 
erate the  segmentation  cavity,  and 
to  differentiate  the  previous  single  layer  into  two.  This  two- 
layered  embryo  is  known  as  a  gastrula  (Fig.  5). 


FIG.  4.  Section  of  an  early 
stage  ef  the  egg  of  Amblystoma 
showing  the  smaller  cells  at  one 
pole,  the  larger  at  the  other, 
and  at  S  the  segmentation 
cavity. 


6  INTRODUCTORY  EMBRYOLOGY. 

The   outer  of  the  two  layers  of  the  gastrula  is  called  the 
ectoderm,  the  inner  the  entoderm.1     The  cavity  bounded  by  the 


FIG.  5.    Diagram   of   a  gastrula  and   of   the  later  closure  of  the  blastopore, 
a,  archenteron;  <£,  blastopore  ;  ec,  ectoderm  ;  en,  entoderm  ;  S,  segmentation  cavity. 

entoderm  is  the  archenteron  (stomach),  and  the  opening  where 
ectoderm  and  entoderm  meet,  and  where  the  archenteron  com- 
municates with  the  external  world,  is  the  blastopore.  Usually 


FlG.  6.  Sagtital  section  of  early  embryo  (late  gastrula)  of  Amblystoma.  b, 
blastopore;  c,  beginning  of  infolding  of  brain;  ^,  entoderm;  ;//,  thickening  of 
ectoderm  for  mouth,  hypophysis,  and  nose  ;  ;/«,  mesoderm  ;  y,  yolk-mass. 

the  blastopore  is  an  elongate  slit,  its  major  diameter  coinciding 
with  the  longitudinal  axis  of  the  future  animal.  Soon  after 
invagination,  the  blastopore  begins  to  close,  the  opposite  lips 

i  In  many  English  works  these  two  layers  are  called  respectively  epiblast  and 
hypoblast,  while  the  mesoderm.  to  be  mentioned  later,  is  called  mesoblast.  There  is  no- 
longer  necessity  for  using  these  terms.  . 


INTRO D UCTOR Y  EMBR YOLOG Y. 


uniting  in  the  median  line.  This  process  of  closure  begins  at 
one  end  and  proceeds  towards  the  other,  the  end  where  the  first 
union  takes  place  being  the  anterior.  In  some  forms  the 
blastopore  never  closes  completely,  but  persists  in  part  as  the 
anus  of  the  adult.  In  those  forms  where  it  closes  completely 
the  anus  later  appears  in  the  line  of  fusion.  Another  landmark 
may  be  noted  here,  —  the  blastopore  closes  along  the  median 
line  of  the  back  ;  and  the  region  of  this  closure  is  known  as  the 
primitive  streak,  the  line  of  closure  being  the  primitive  groove. 
From  the  region  of  the  blastoporal  lips  (primitive  streak) 
there  next  grows  into  the  segmentation  cavity,  on  either  side, 
a  third  layer,  —  the  meso- 
derm.  In  several  forms 
(Fig.  7)  this  mesoderm 
clearly  arises  as  an  out- 
growth from  the  entoderm 
in  the  shape  of  a  double 
fold,  its  walls  bounding  a 
cavity  (coelom)  which  at 
first  is  connected  with  the 
archenteron.  Later  the 
connection  between  these 
coelomic  pouches  and  the 
archenteron  is  lost,  the  lips 
of  the  outgrowth  fusing, 
and  then  the  mesoderm 
completely  separates  from 
the  entoderm.  In  other 
cases  the  mesoderm  ap- 
pears as  a  solid  outgrowth 
from  the  same  point,  and 


FIG.  7..  Transverse  section  of  Ambly- 
stonia  embryo  showing  formation  of  meso- 
derm (mesothelium).  a,  archenteron  ; 
c,  coelom  ;  ec,  ectoderm  (outer  layer)  ;  en, 
ectoderm  (nervous  layer);  ;//,  medullary 
plate  ;  n,  notochprdal  cells  ;  /,  parietal 
layer  of  mesothelium  ;  s,  remains  of  seg- 
mentation cavity;  z>,  visceral  (splanchnic) 
layer  of  mesothelium. 


later  it  splits  so  as  to  form 
a  coelom  comparable  to  that  first  described.1  The  result  in 
either  case  is  that  the  segmentation  cavity  is  still  farther 
reduced  by  the  extension  into  it,  on  either  side  of  the  embryo, 
of  a  flattened,  mesodermic  sac.  In  this  sac  two  walls  can  be 


1  The  type  of  coelom  in  the  first  case  is  called  an  enteroccele  ;  in  the  second  a 
schizocoele. 


8  INTRODUCTORY  EMBRYOLOGY. 

distinguished  :  the  one  turned  towards  the  ectoderm  is.  called 
the  somatic  or  parietaMayer ;  the  one  facing  the  entoderm  is 
the  splanchnic  or  visceral  layer. 

Besides  this  mesoderm  arising  thus  as  a  continuous  out- 
growth, another  type  of  mesoderm  also  invades  the  segmenta- 
tion cavity.  This  arises  by  the  migration  or  inwandering  into 
this  space  of  single  cells,  which  may  separate  themselves  from 
either  entoderm  or  mesoderm  ;  or  in  some  instances,  as  recent 
investigations  tend  to  show,  from  the  ectoderm  as  well.  Since 
these  two  types  of  mesoderm  differ  in  their  origin,  and,  as  will 
be  seen  later,  in  their  character  and  fate,  they  have  been  given 
different  names.  That  mesoderm  which  bounds  the  coelomic 
cavities  and  all  parts  formed  from  it  is  called  mesothelium  ; 
that  which  arises  from  the  scattered  immigrant  cells  is 
mesenchyme.  — ^ 

At  this  point,  where  the  four  germ-layers  of  the  embryo  are 
differentiated  from  each  other,  it  will  be  interesting  to  state 
what  portions  of  the  adult  vertebrate  are  derived  from  each. 

The  ectoderm  gives  rise  to  the  outer  portion  (epidermis)  of 
the  skin,  the  outer  layer  of  scales,  hair,  feathers,  the  enamel  of 
the  teeth,  nails,  claws,  true  horn,  and  the  essential  parts  of  all 
sensory  and  nervous  structures. 

The  entoderm  develops  into  the  lining  of  the  alimentary 
canal  and  the  various  cavities  —  gills,  lungs,  liver,  pancreas  — • 
connected  with  it  ;  also  to  the  notochord,  and  possibly  to  the 
lining  of  the  blood-vessels. 

From  the  mesothelium  arises  the  lining  of  the  body  cavity, 
reproductive  and  excretory  organs,  and  the  voluntary  muscles 
(including  the  muscles  of  the  heart). 

The  mesenchyme  produces  the  deeper  layers  of  the  skin, 
the  lower  portions  of  scales,  and  the  dentine  of  the  teeth ;  invo"P 
untary  muscles,   connective   tissue,   fat,   cartilage,  bone,   blood^ 
and  lymph  corpuscles. 

From  the  point  where  the  germ-layers  are  outlined  the  de- 
velopment must  be  traced  in  two  different  directions.     One  line 
follows  out  the  differentiation  of  the  cells  and  their  grouping 
into  tissues  ;  the  other  traces  the  development  of  the  various 
,  organs  of  the  adult. 


EPITHELIAL    TISSUE. 


HISTOLOGY. 

Histology  deals  with  the  minute  structure,  and  especially 
with  the  characters  of  the  cells  and  the  tissues  arising  from 
them.  In  the  adult  occur  cells  varying  in  shape  and  size,  and 
adapted  for  various  functions  ;  those  cells  which  are  alike 
grouped  together  into  tissues.  A  tissue,  then,  may  be  defined 
as  an  aggregate  of  similar  cells,  together  with  a  varying  amount 
of  intercellular  substances,  usually  produced  by  the  cells  them- 
selves. The  cells  themselves  are  the  living  portions  of  the 
tissue ;  the  intercellular  substance,  by  its  amount  and  character, 
being  directly  influential  in  determining  the  nature  of  the 
tissue.  Tissues  may  be  solid  or  fluid  ;  may  form  thin  sheets 
or  thick  masses.  All  tissues  can  be  grouped  under  four  heads, 
—  epithelial,  nervous,  muscular,  and  connective. 

Epithelial  tissues  are  the  primitive  tissues.  An  epithelium 
is  a  layer  of  cells  covering  any  free  surface  on  or  in  the  body. 
Thus  in  the  gastrula  both  ectoderm  and  entoderm  are  epithelia, 
since  the  one  covers  the  outside,  while  the  other  lines  the 
archenteron.  The  mesothelium  is  also  epithelial l  since  it  lines 
the  cavity  of  the  ccelom.  Epithelia  are  classified  according  to< 
shape,  arrangement,  or 
character  of  the  cells.  In 
cubical  or  columnar  epi- 
thelium the  cells  have 
shapes  corresponding 
to  their  names  ;  in 
pavement  epithelium  the 
cells  are  greatly  flat- 
tened, so  that  each 
one,  while  very  thin,  covers,  comparatively,  a  large  amount 
of  surface.  Epithelia  are  simple  when  the  cells  are  arranged 
in  a  single  layer;  stratified  when  they  form  several  layers. 
In  some  cases  the  epithelial  cells  may  bear  on  their  free  sur- 

1  The  term  epithelium  is  sometimes  restricted  to  those  layers  on  the  outer  surface  of 
the  body,  or,  like  the  epithelium  of  the  lungs  and  stomach,  connected  with  the  exterior.  The 
similar  cells  in  the  closed  cavities,  like  the  body  cavity  or  the  blood-vessels,  are  then  called 
endothelia.  The  distinction  is  of  little  importance. 


FIG.  8.     Epithelia;  A,  columnar;  B,  pave- 
ment, in  perspective ;   C,   cubical ;  Z>,  stratified. 


10 


HISTOLOGY. 


faces  minute  vibratile  hair-like  processes  (cilia),  whence  this 
type  is  called  ciliated  epithelium.  Again,  epithelia  may  be 
grouped  according  to  function,  and  then  cuticular,  sensory,  and 
glandular  epithelia  may  be  recognized.  Epithelia  may  give  rise 
to  important  structures,  such  as  hair,  feathers,  scales,  enamel  of 
the  teeth,  etc.,  which  will  be  mentioned  in  the  proper  places. 

Nervous  tissue  arises  from  the  ectoderm,  and  hence  from 
epithelium.  It  has  for  its  purpose  the  recognition  and  trans- 
ferrence  of  impulses,  the  perception  of  sensations,  and  the  pro- 
duction of  other  impulses  which  shall  affect  nervous  or  other 
tissues.  It  has  for  its  essential  constituent  nerve  cells,  to  which 
are  usually  added  other  cells  of  a  supportive  nature.  Nerve 
cells  (or  ganglion  cells)  consist  of  a  central  nucleated  body 
from  which  radiate  one  or  more  protoplasmic  processes  which, 


FIG.  9.  Different  forms  of  nerve  cells  from  cat.  A,  pyramid  cell 
from  cerebrum  ;  B,  cell  from  spinal  cord  ;  D,  unipolar  cell  from  spinal  gan- 
glion ;  C,  glia  cell  from  spinal  cord ;  a,  axis  cylinder. 

after  a  longer  or  shorter  course,  break  up  into  minute  branches 
or  fibrillations.  It  must,  however,  be  kept  in  mind  that  these 
processes  are  really  parts  of  the  cell,  although  the  term  cell  is 
frequently  restricted  to  the  central  mass,  while  the  processes 
are  called  nerve  fibres,  etc.  When  a  nerve  cell  has  two  proto- 
plasmic processes  it  is  spoken  of  as  bipolar  ;  when  more  than 
two  as  multipolar.1  One  of  these  processes  is  of  considerable 

i  In  some  cases  '  unipolar '  nerve  cells  are  found ;  but  the  process  in  these  is  soon 
found  to  divide,  its  halves  going  off  at  right  angles  to  the  previous  course,  thus  showing 
that  these  cells  are  really  bipolar. 


NERVOUS   TISSUE. 


II 


length,  and  is  known  as  an  axis  cylinder  or  axon ;  the  others 
are  shorter,  and  as  they  soon  break  up  into 
minute  branches,  they  are  called  dendrites. 
In  most  cases  the  axis  cylinders  have  a  similar 
method  of  termination.  Recent  investigations 
show  that  the  only  connection  between  nerve 
cells  consists  in  an  interlacing  of  these  fibril- 
Ice  ;  two  nerve  cells  never  join. 

The  axis  cylinder  is  the  essential  part  of 
a  nerve  fibre.  Of  these  fibres  two  kinds  are 
to  be  recognized.  In  the  medullated  fibres 
the  axis  cylinder  is  surrounded  by  a  medul- 
lary sheath  of  a  peculiar  substance  (myelin) 
rich  in  fat.  This  sheath,  it  is  to  be  noted, 
usually  stops  before  the  end  of  the  axis  cylin- 
der, and  in  most  cases  it  is  not  continued  to 
the  central  mass  of  the  cell.  In  the  non-medul- 
lated  fibres  the  sheath  is  lacking,  and  only  the 
axis  cylinder  is  found.  Both  medullated  and 
non-medullated  fibres  may  have  a  second  sheath 
(the  neurilemma  or  sheath  of  Schwann)  de- 
rived from  the  connective  tissue  (see  below), 
and  containing  scattered  nuclei. 

Nervous  tissue  is  made  up  of  these  nerve 
cells.     In  a  nerve  proper  we  have  but  a  bundle 
of   nerve  fibres  (axis  cylinders,  medullated  or         FIG.  10.     Por- 
non-medullated)  bound  together  by  connective    tions  of  medullated 

...  -  nerve    fibres  (from 

tissue,  while  the  bodies  of  the  cells  are  absent.  Martin).  The 
These  nerves  are  but  conducting  trunks,  bear-  medullary  sheath, 
ing  impulses  to  or  from  the  central  portion  of  stamed  black  bY 

.  ,,         „  .  ...        osmic  acid,  is  inter- 

the  cell.  From  their  color,  those  parts  which  rupted  at  R^  tne 
are  formed  entirely  of  nerve  fibres  are  called  nodes  of  Ranvier, 
the  white  matter.  The  cell  bodies,  together  across  which  the 

..,       ~,  .        .    .  -  .          axis     cylinder     ex- 

with  fibres,  dendrites,  etc.,  unite  to  form  the  tends>  Outside  the 
gray  matter,  which  may  be  aggregated  in  medullary  sheath  is 
smaller  centres  (ganglia)  or  in  larger  continu-  the  Oesenchyme) 

,         ,  1-1  i       sheath  of  Schwann, 

ous  tracts,  as  in  the  brain  and  spinal  cord.  the  nudei  of  which 
In  these  parts  occur  certain  supporting  cells  are  seen  at  c. 


12 


HISTOLOGY. 


(neuroglia)  derived  from  the  ectoderm,  but  lacking  entirely  in 
nervous  properties.  These  glia  cells  are  extensively  branched, 
their  branches  running  between  fibres  and  cell  bodies  (Fig.  9,  £7). 
Muscular  tissue  is  the  special  contractile  element  in  the 
body.  It  is  of  two  kinds,  different  in  origin,  structure,  and  ac- 
tion. The  mesenchyme  gives  rise  to  the  smooth 
muscle.  This  consists  of  long  spindle-shaped  cells, 
each  usually  containing  a  single  nucleus,  and  being 
marked  with  fine  longitudinal  lines.  These  cells  may 
occur  singly,  or  may  be  arranged  in  small  bundles 
or  thin  sheets ;  and  in  all  cases  they  are  not  under 
control  of  the  will,  a  fact  that  gives  rise  to  the  name, 
involuntary  muscles,  often  applied  to  them.  Smooth 
muscular  tissue  is  slow  in  its  action. 
Striped  muscular  tissue,  on  the 
other  hand,  is  derived  from  the  mes- 
othelium  by  modification  from  the 
muscle  plates,  to  be  described  later. 
It  occurs  usually  in  larger  masses 
than  does  the  mesenchymatous 
muscular  tissue,  and  is  (except  in 
the  case  of  the  heart)  under  control 
of  the  will.  This  striped  tissue  consists  either 
of  separate  cells  (heart  muscles)  or  cf  usually 
long  cylindrical,  so-called  primitive  fibres,  each 
of  which  contains  several  nuclei ;  i.e.,  is  syn- 
cytial.  In  these  primitive  fibres  the  bulk  of 
the  protoplasm  has  been  altered  into  a  strongly 
contractile  substance  marked  with  fine  trans- 
verse lines.  Around  each  fibre  is  a  struc- 
tureless envelope,  the  sarcolemma ;  while  the 
fibres  are  bound  together  into  muscle  bun- 
dles by  means  of  connective  tissue  envelopes 
(perimysium)  bearing  nerves  and  blood-ves- 
sels, and  continuous  with  the  tendons  and  („)  an(j  the  sarco- 
fascia  by  which  the  muscles  are  attached  to  lemma,  s,  where  the 
other  structures.  The  nuclei  are  oval,  with  muscle  fibre  .is  torn- 

From  Hertwig,  after 

their  long  axes  parallel  to  the  direction   ot      Gegenbaur.        «• 


FIG.  ii. 

Smooth 
muscle 
fibres. 


FIG.    12.       Cross 


THE   CONNECTIVE    TISSUES. 


the  fibres.  In  the  mammals  they  are  placed  upon  the  periphery 
of  the  fibres  (Fig.  12,  n),  in  the  lower  vertebrates  near  the  cen- 
tre. The  muscles  of  the  heart  agree  in  origin  and  cross-band- 
ing with  the  voluntary  muscles,  but  differ  in  being  cellular  rather 
than  syncytial,  and  in  being  removed  from  the  control  of  the 
will.  All  cross-banded  muscles  are  capable  of  rapid  contrac- 
tions. 

The  connective  tissues  are  all  of  mesenchymatous  origin,  and 
are  characterized  by  a  great  development  of  the  intercellular 
substance,  which 
is  usually  a  pro- 
duct of  the  cells. 
They  are  the  sup- 
porting  tissues  of 
the  body,  and 
vary  accordingly 
as  this  intercellu- 
lar substance  va- 
ries, and  may 
correspondingly 
be  grouped  under 
Several  subheads,  FIG>  ^  Fibrous  non.eiastic  connective  tissue 

the  principal  ones  (from  Martin). 

being  enumerated  below. 

In  fibrous  connective  tissue  (white  or  non-elastic  tissue)  the 

cells  are  branched  or  spindle-shaped,  and  the  intercellular  sub- 
stance is  more  or  less  fibrous, 
the  fibres  being  parallel,  in- 
terlaced, or  in  a  network,  so 
that  there  result  sheets,  mem- 
branes, or  bundles,  accord- 
ingly as  the  part  to  be  played 
varies.  In  some  cases  this  tis- 
sue is  loose  (areolar  tissue), 
such  as  is  found  between  the 
skin  and  deeper  parts  ;  at 


FIG.    14.     Fat.     o,  oil  globules  in  the 
connective  tissue  cells. 


other  times  it  is  much  firmer, 
as  in  the  case  of  tendons.     This  type  of  tissue  also  gives  rise  to 


HISTOLOGY. 


FIG.  15.     Elastic  tissue 
(from  Martin). 


fat  (adipose  tissue)  by  the  deposition  of  oil  in  the  protoplasm 
of  the  cells.  In  elastic  tissue  (yellow 
connective  tissue)  the  intercellular  fibres 
are  larger,  and  elastic  in  character. 

In  cartilage  the  intercellular  sub- 
stance (here  called  the  matrix)  is  more 
solid  and  firmer.  It  varies  considerably 
in  abundance,  and  in  proportion  to  its 
amount  the  cartilage  gains  as  a  support- 
ing tissue.  When  the  matrix  is  homo- 
geneous, the  result  is  hyaline  cartilage  ; 
but  it  may  be  traversed  by  fibres  of 
white  or  yellow  connective  tissue,  thus 
producing  fibrous  or  elastic  cartilage. 
The  cells  of  cartilage  are  circular,  oval, 
or  fusiform  in  outline  ;  but  they  send  out 
very  fine  protoplasmic  processes  which 
traverse  the  matrix,  thus  connecting  all  parts.  Cartilage  in- 
creases in  size  in  three  ways,  —  by  addition  of  new  cells  to  the 
outside,  by  increase  in  the  amount  of  the  matrix,  and  by  division 
of  the  cells  in  the  cartilage 
itself.  In  almost  every  sec- 
tion of  cartilage  several  gen- 
erations of  cells  may  be 
readily  traced  by  observing 
the  capsules  surrounding 
them.  Cartilage  is  very 
closely  related  to  bone,  and 
is  frequently  converted  into 
that  more  solid  substance  by 
a  change  (ossification)  in  its 
matrix.  Cartilage  may  also 
be  calcified  by  the  deposition 
of  lime  upon  its  surface. 
Calcified  cartilage  and  bone  are  entirely  distinct. 

In  bone  this  matrix  consists  of  an  organic  basis  combined 
with  salts  of  lime  (chiefly  carbonate  and  phosphate)  ;  while  car- 
tilage is  usually  solid,  bone  is  traversed  by  tubes  (Haversian 


Wim 


FIG.  1 6.    Hyaline  cartilage,  the 
matrix  dotted. 


THE   CONNECTIVE    TISSUES. 


canals)  bearing  nutrient  vessels,  etc.  Arranged  in  layers  con- 
centric to  these  canals  or  parallel  to  the  surface  of  the  bone  are 
the  cells,  each  occupying 
a  space  (lacuna)  in  the 
dense  matrix.  These 
cells  are  connected  by 
fine,  branching,  proto- 
plasmic processes,  which 
run  in  minute  tubules 
(canaliculi)  through  the 
layers  (lamellae)  of  the 
matrix.  Both  cartilage 
and  bone  are  enveloped 
in  a  layer  of  fibrous  con- 
nective tissue,  called  re- 

FIG.   17.     Bone.      A,  piece  of   a  long  bone 


Bone. 

showing  the  appearance  under  low  power  in 
longitudinal  and  cross  sections;  B,  a  transverse 
section  of  three  lamellae  surrounding  an  Haver- 
sian  canal,  from  a  slice  of  dried  bone;  c,  bone 
corpuscles ;  fit,  canaliculi ;  h,  Haversian  canal ; 
/,  lamellae. 


spectively  perichondrium 
and  periosteum. 

Many  bones,  as  has 
just  been  said,  pass 
through  a  cartilage  stage 
in  their  history,  the  gen- 
eral outlines  being  built  up  in  that  more  yielding  substance. 
Later  the  matrix  is  dissolved  little  by  little,  and  is  replaced  by 

the  lime  salts,  the 
cells  (osteoblasts) 
becoming  enclosed 
in  the  hardened  sub- 
stance. Such  bones 
are  called  cartilage 
bones.  Other  bones, 
however,  have  no 
cartilage  stage,  but 
arise  from  the  calci- 
fication of  the  inter- 
cellular substance  of 
membranes,  and 
these  are  called  membrane  bones.  In  either  case  the  process 
of  ossification  proceeds  from  fixed  spots  (centres  of  ossification) 


FIG.  1 8.  Development  of  membrane  bone 
(mandible  of  pig).  Around  the  (black)  bone  are 
numerous  osteoblasts,  some  of  which  are  included  in 
the  bony  substance. 


i6 


HISTOLOGY. 


extending  gradually  in  all  directions  until  the  final  result  is 
much  the  same.  In  structure  cartilage  bones  and  membrane 
bones  are  very  similar,  but  the  differences  in  the  history  is  very 
important,  as  will  be  seen  later  in  dealing  with  the  skeleton. 

Closely  allied  to  bone  is  the  dentine  of  teeth  and  scales,  the 
chief  differences  lying  in  the  greater  density  of  the  intercellular 
substance,  and  in  the  fact  that  the  dentine-producing  cells 
(odontoblasts)  do  not  become  included  in  the  solid  structure. 
The  same  fine  protoplasmic  processes  of  the  cells  exist,  lying  in 
dentinal  canals  which  pursue  a  nearly  parallel  course. 

Blood  and  lymph  are  connective  tissues,  with  a  fluid  intercel- 
lular substance  (plasma)  in  which  float  the  cells.  In  lymph  the 

cells  are  all  of  one  type,  known  as  leu- 
cocytes, white  in  color,  and  possessed 
of  marked  amoeboid  powers.  In  blood, 
besides  the  leucocytes  (white  corpus- 
cles), there  are  also  numerous  red  cor- 
puscles, the  source  of  color  of  the  blood. 
These  red  corpuscles  have  no  amoeboid 
powers,  but  are  merely  the  means  of 
transferrence  of  oxygen  and  carbon  di- 
oxide l  to  and  from  the  tissues.  In 
the  lower  vertebrates  the  red  corpus- 
cles are  oval  and  nucleated  ;  in  the  mammals  the  nuclei  are  lost, 
and  the  corpuscles  are  usually  biconcave,  circular  disks.  The 
blood  plaques  may  also  be  mentioned. 


FIG.  19.  a,  b,  <:,  red  blood 
corpuscle  of  man;  d,  white 
corpuscle  of  man;  e,  red  cor- 
puscle of  frog. 


Carbon  dioxide  is  also  carried  by  the  plasma. 


VDERMAL    ORGANS. 


MORPHOLOGY    OF    THE    ORGANS    OF 
VERTEBRATES. 


FROM  the  point  where  the  four  germ-layers  are  clearly  dif- 
ferentiated from  each  other,  we  have  now  to  trace  the  various 
derivatives  of  each  ;  but  it  must  be  kept  in  mind  not  only  that 
various  organs  are  in  the  process  of  development  at  the  same 
time,  while  the  necessities  of  treatment  demand  that  they  be 
arranged  in  sequence,  but  that  two  or  more  layers  not  infre- 
quently contribute  to  the  same  organ.  In  such  cases  the  organ 
is  described  in  connection  with  that  layer  which  is  most  promi- 
nent or  most  important  in  its  structure.  In  the  following  account 
the  stages  of  development  are  traced  only  with  such  detail  as  is 
necessary  for  a  clear  interpretation  of  the  adult  structure.  For 
more  extended  accounts  the  student  must  go  to  the  embryologi- 
cal  manuals  and  special  memoirs. 

ENTODERMAL    ORGANS. 

The  differentiation  of  the  entoderm  by  invagination  has  been 
described  (p.  5).  By  this  process  of  gastrulation  a  layer  of 
entoderm  cells  comes  to  lie  inside  the  other  or  ectoderm  cells, 
and  by  the  closure  of  the  blastopore  (usually  complete)  it  assumes 
the  form  of  a  sac,  the  cavity  of  which  is  the  archenteron.  As 
the  embryo  elongates,  the  sac  forms  an  elongate  tube.  In  the 
middle  line  of  its  dorsal  wall  a  cord  of  cells,  lying  between  the 
outgrowing  ccelomic  pouches  (Fig.  7,  ;/),  becomes  constricted 
off  from  the  rest,1  and  occupies  a  position  between  the  other 
entodermal  structures  and  the  nervous  system  (Figs.  7  and  20). 
This  rod  is  the  notochord,  the  subsequent  history  of  which  is 
given  in  connection  with  the  skeleton. 

The  rest  of  the  entoderm,  after  the  formation  of  the  noto- 

1  In  a  few  forms  (e.g.,  Amblystoma)  this  cord  is  at  first  tubular  ;  later  its  lumen  is  lost. 


i8 


MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


chord,  gives  rise  to  the  lining  (epithelium)  of  the  digestive 
canal  (alimentary  tract)  and  its  appendages,  and  of  the  respira- 
tory organs  (gills  and  lungs). 

The  first  step  in  the  differentiation  of  the  alimentary  struc- 
tures is  the  formation  of  an  outpocketing  on  the  ventral  side,  the 

beginning  of  the  liver 
(Fig.  20,  /).  This  oc- 
curs some  distance  in- 
front  of  the  middle  of 
the  body,  and  divides 
the  alimentary  canal 
into  pre-  and  post- 
hepatic  portions.  The 
post-hepatic  portion 
gives  rise  to  the  intes- 


FIG.  20.  Longitudinal  section  of  Amblystoma 
embryo.  /z,  hypophysis;  ht>  heart,  its  endo- 
thelial  walls  not  shown ;  z,  infundibulum  ;  in, 
intestine;  /,  liver;  m,  mesenchyme ;  n,  noto- 
chord ;  /,  pineal  outgrowth ;  pc,  pericardial  cavity. 


tine  and  its  various  di- 
visions, including  the 
pancreas  ;  from  the  pre- 
hepatic  region  are  developed  the  pharynx,  with  the  respiratory 
structures,  the  gullet,  and  the  stomach. 

The  Mouth.  —  Besides  these  entodermal  structures,  the  ali- 
mentary tract,  as  usually  considered,  embraces  as  well  the  cavity 
of  the  mouth,  the  lining  of  which  is  ectodermal  in  origin.  The 
mouth  arises  as  an  inpushing  or  involution  of  the  ectoderm  1  at 
the  anterior  end  of  the  ventral  surface  of  the  body.  The  in- 
pushing  usually  takes  the  form  of  a  cup,  the  blind  end  of  which 
impinges  directly  upon  the  closed  anterior  end  of  the  alimentary 
canal  proper,  thus  forming  a  double  partition  between  the  two 
(Fig.  55).  These  two  membranes,  one  ectodermal  the  other 
entodermal  in  origin,  fuse,  and  then  an  opening  breaks  through, 
thus  placing  the  whole  in  communication.  From  this  ectodermal 
oral  invagination  or  stomodaeum  are  developed  the  lips,  teeth, 
tongue,  and  glands. 

The  lips  bound  the  opening  of  the  mouth.  In  all  the  lower 
vertebrates  they  are  merely  folds  of  epithelium,  or,  as  in  turtles 

1  In  some  forms  this  inpushing  is  plainly  a  paired  structure,  a  fact  which  adds  no 
little  weight  to  the  view  which  regards  the  vertebrate  mouth  as  having  arisen  from  the 
coalescence  of  a  pair  of  gill  slits. 


TEETH.  19 

and  birds,  they  may  be  entirely  absent.  In  the  mammals  fleshy 
lips  moved  by  muscles  first  occur,  and  even  here  they  are  lacking 
in  monotremes  and  cetaceans.  In  turtles  and  birds  the  edges  of 
the  jaws,  and  to  a  greater  or  less  extent  the  roof  of  the  mouth, 
is  covered  with  a  cornified  epithelium  forming  the  so-called  beak, 
and  the  same  is  true  of  the  adult  monotremes.  The  surface  of 
this  may  be  thrown  into  folds  for  the  purpose  of  crushing  the 
food,  but  these  structures  are  not  to  be  compared  with  true  teeth. 
Teeth.  —  In  the  formation  of  teeth  two  layers,  ectoderm  and 
mesenchyme,  are  concerned.  The  epithelium  lining  the  mouth 
becomes  inpushed  into  the  deeper  layers,  where  teeth  are  to  be 
formed  (Fig.  21).  In  the  lower  vertebrates  there  is  a  separate 
inpushing  for  each  tooth,  but  in  the  mammals  there  is  a  con- 
tinuous ingrowth, —  the  dental  ridge.  In  other 
respects  the  features  of  development  are  essen- 
tially the  same  in  all.  The  ingrowth  is  to  be 
regarded,  morphologically,  as  vesicular ;  and  the 
deeper  wall  of  the  vesicle  becomes  pushed  in- 
side the  other,  so  that  there  results  a  two- 
walled  cup,  the  cavity  of  which  becomes  filled 
with  mesenchyme.  The  cells  of  the  inner  layer 

*  FIG.  21.    Tooth 

become  columnar  and  form  the  enamel  organ ;  germ  Of  Ambiysto- 
the  immigrant  mesenchyme  cells  constitute  a  ma.  d,  derma ;  e, 
dental  papilla,  the  external  cells  of  which  are  ePidermls;  °>  en~ 

amel  organ ;  /,  den- 

known    as    odontoblasts,   from    their    power   of  tal  papjj]a- 
secreting    a   bone-like    substance,    the    dentine 
or  ivory  of  the  tooth.     The  inner  surface  of  the  enamel  organ 
likewise  secretes  a  cup  of  denser  substance,  enamel,  upon  the 
outer  face  of  the  dentine. 

By  growth  of  the  deeper  portions  (dentine)  the  enamel  cov- 
ered tip  or  crown  of  the  tooth  is  forced  up  through  the  epithelial 
layers  so  that  it  comes  into  position  for  use.  The  deeper  por- 
tion or  root  contains  a  central  or  pulp  cavity,  in  which  are  remains 
of  the  mesenchyme,  together  with  nerves  and  blood-vessels,  these 
together  forming  the  pulp.  In  the  mammals  the  root  is  cov- 
ered by  a  second  coat,  the  cement,  formed  by  the  surrounding 
tissues  (Fig.  22). 

In  the  lower  forms  the  process  of  tooth  formation  may  con- 


2O       MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


tinue  for  a  long  time,  even  through  life,  new  teeth  thus  arising 
to  make  good  the  loss  of  others.  In  the  mammals,  however, 
there  are  at  most  but  two  of  these  sets  of  teeth,  —  a  temporary 
or  milk  dentition,  and  a  second  or  permanent  dentition,  the  details 
of  which  are  given  in  connection  with  that  group. 

In  the  lower  vertebrates  teeth  may  appear 
in  any  part  of  the  mouth  where  there  are 
solid  parts  (bones  or  cartilages)  to  support 
them.  Thus  in  fishes  and  amphibia  we  may 
find  them  not  only  along  the  edges  of  the 
jaws,  but  upon  any  of  the  bones  which  lie 
in  the  walls  of  the  oral  cavity.  In  the  higher 
vertebrates  they  are  confined  solely  to  the 
edges  of  the  jaws. 

Teeth  are  very  variable  in  shape,  a  fact 
largely  correlated  with  differences  in  food. 
In  the  lower  vertebrates  all  of  the  teeth  of 
an  individual  are  closely  similar  to  each  other. 
This  is  the  hgrnodont  condition ;  the  hetero- 
dont  dentition  appears  in  the  mammals,  and 
occasionally  in  the  lower  groups,  where  the 
teeth  in  the  different  regions  of  the  mouth 
are  of  different  shapes.  Usually  in  the  lower 
vertebrates  each  tooth  possesses  but  a  single 
root  and  a  single  cusp ;  while  in  the  mam- 
mals, besides  these  simple  teeth,  there  are 
others,  with  two,  three,  or  several  roots,  the 
crowns  also  showing  a  corresponding  duplica- 
tion of  parts. 

In  the  elasmobranchs  the  teeth  rest  upon 

but  are  not  firmly  united  to  the  skeletal  parts.  In  the  other 
ichthyopsida  they  are  usually  firmly  united  to  the  bones  of  the 
mouth  by  continuous  growth,  and  the  same  is  true  of  many 
reptiles.  In  others  they  may  be  implanted  in  sockets  (alveoli) 
in  the  jaws,  a  condition  which  is  universal  in  the  mammals. 

Besides  these  true  or  calcified  teeth,  horny  teeth  occur  here 
and  there,  as  in  the  cyclostomes,  where  the  oral  hood  and  the 
tongue  are  armed  with  such  structures  resting  upon  epithelial 


FIG.  22.  Dia- 
grammatic section  of 
mammalian  incisor 
tooth,  c, cement;  dy 
dentine ;  <?,  enamel; 
/,  pulp  cavity. 


ORAL    GLANDS.  21 

papillae,  or  in  the  larvae  of  the  anura,  where,  besides  the  horny 
jaws,  there  are  numbers  of  minute  cornified  teeth.  Mention 
may  also  be  made  here  of  the  oesophageal  teeth  of  the  snake 
RJiachiodon,  which  consist  of  ventral  processes  of  the  cervical 
vertebrae,  each  with  its  cap  of  enamel.  These  project  through 
the  dorsal  wall  of  the  oesophagus,  and  serve  to  cut  open  the 
eggs  upon  which  these  reptiles  feed.  . 

Tongue.  — The  tongue  is  primitively  a  fleshy  fold  of  the  floor 
of  the  mouth,  supported  to  a  greater  or  less  extent  by  some  of 
the  elements  of  the  first  visceral  (hyoid)  arch  (see  skeleton). 
In  the  fishes  this  tongue  is  without  its  own  muscles,  and  can 
be  moved  only  in  connection  with  the  branchial  arches.  In  the 
cyclostomes,  on  the  other  hand,  lingual  muscles  —  protractors  and 
retractors  —  of  considerable  size  appear,  while  in  the  amphibia 
and  higher  groups  similar  muscles  are  usually  well  developed!1 
In  the  amphibia  the  tongue  is  fastened  by  its  ventral  surface  or 
its  anterior  end.  In  the  reptiles,  on  the  other  hand,  it  is  fastened 
behind.  In  this  group,  as  in  the  birds,  it  is  usually  horny,  with 
few  intrinsic  muscles,  and  in  these  the  hyoid  becomes  more 
modified  as  a  lingual  skeleton.  In  the  mammals  the  tongue 
reaches  its  highest  development,  with  very  considerable  varia- 
tions of  form.  Beneath  the  tongue,  in  many  mammals,  is  a 
small  fold,  the  sublingua,  which  is  regarded  by  Gegenbaur  as 
homologous  with  the  tongue  of  the  lower  forms,  the  mammalian 
tongue  being  a  structure  peculiar  to  that  group. 

Oral  Glands.  —  Glands  connected  with  the  oral  cavity  first 
appear  in  the  amphibia,  where  in  the  epithelium  occur  numerous 
mucous  glands,  the  secretion  of  which  moistens  the  lining  of 
the  mouth.  Besides  these,  the  higher  amphibia  have  a  larger 
internasal  gland  opening  in  the  palate  region.  In  the  reptiles 
glands  are  more  numerous,  occurring  on  and  beneath  the  tongue, 
and  along  the  margins  of  the  jaws.  From  these  latter  are 
developed  the  poison  glands  of  snakes  ;  while  if  the  lizard  Helo- 
derma  be  poisonous,  its  poison  is  secreted  from  the  large  sub- 
lingual  gland.  In  the  birds  the  glands  are  not  so  numerous, 
those  of  the  tongue,  palatal  region,  and  angle  of  the  mouth, 
being  most  conspicuous.  In  the  mammals  three  pairs  of  glands 

1  Occasionally  (PiJ>a,  Dactylethra)  a  tongue  is  not  formed. 


22        MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRATES. 


occur,  —  a  sublingual,  a  submaxillary,  and  a  parotid.  None  of 
these  are  poisonous ;  but  the  saliva  which  they  secrete  is  for 
moistening  the  food  during  mastication,  and  for  the  conver- 
sion of  starch  into  sugar. 

From  the  pharyngeal  region  are  devel- 
oped the  respiratory  organs,  —  gills  and 
lungs,  —  as  well  as  certain  other  struc- 
tures, the  thyroid  gland,  thymus  gland, 
etc. 

Gills.  —  Gills  arise  as  a  series  of  paired 
or  bilateral  outpushings  of  the  entodermal 
lining  of  the  pharynx.  These  push  out 
through  the  mesodermal  and  mesenchyma- 
tous  tissues  until  they  reach  the  ectoderm 
on  the  sides  of  the  neck.  The  two  layers 
now  fuse,  and  then  an  opening  is  formed 
at  the  point  of  fusion,  so  that  there  arise  a 
series  of  openings  (gill-,  branchial-,  or  vis- 
ceral-clefts) on  either  side,  connecting  the 
pharyngeal  cavity  with  the  external  world. 
In  the  septa  between  the  clefts  are  devel- 
oped skeletal  structures  (gill-  or  branchial- 
arches,  see  skeleton),  and  also  blood-ves- 
sels. From  the  walls  of  the  clefts  develop 
vascular  leaves  or  filaments,  the  gills  proper. 
These  are  arranged  on  the  anterior  and  pos- 
terior walls  of  the  clefts,  those  on  a  side 
constituting  a  demibranch. 

The  number  of  gill  pouches  differ  in 
different  groups.  In  Bdellostoma  a  (cy- 
clostome)  there  may  be  14  pairs  ;  in  the 
notidanid  sharks  7  or  8  ;  in  other  sharks  6, 
and  from  this  down  to  5  in  reptiles,  and  4  in  mammals.  In  the 
th^opsida  all,  or  nearly  all,  of  these  pouches  break  through 
as  descriBeH  above,  but  in  the  amniotes  but  one  or  two  open 
to  the  exterior ;  the  statements  regarding  the  mammals  being 
conflicting.  In  the  jimniotes  these  gill  pouches  or  clefts  never 
develop  gill  filaments  ;  ancT  in  the  adult  all  traces  of  them,  except 


FIG.  23.  Horizon- 
tal section  through  head 
and  pharyngeal  region  of 
Acanthias  embryo,  show- 
ing the  gill  slits.  £, 
blood-vessels;  <",  coelomic 
cavities  of  gill  arches ; 
/,  developing  gill  fila- 
ments; //,  hypophysis; 
«,  notochord ;  0,  oculo- 
motor nerve ;  /,  pharynx  ; 
s,  spiracular  cleft;  /,  first 
(mandibular)  head  cav- 
ity; I-V,  gill  clefts. 


RESPIRATORY    ORGANS. 


of  the  first,  are  lost.     Their  presence  in  this  group  can  only 

be  explained  as  inheritances  from  branchiate  ancestors.     The 

first   gill  pouch  in  the  anura  and 

the  higher  groups  form  the  Eusta- 

chian  tube  (see  ear). 

In  elasmobranchs  and  some  ga- 
noids the  anterior  visceral  cleft  is 

smaller  than  the  others,  and  opens 

on  the  top  of  the  head.    This  spir- 
acle bears  well-developed    gills  in 

the    lowest    sharks    (notidanidae), 

but  in   others  it  may  have  but  a 

vascular  network  in  its  walls.     In 

ganoids  and  embryonic  teleosfs  it 

has  a  gill-like  structure ;  but  it  is 

here  termed  a  pseudobranch,  since 

it  receives  arterial  blood  from  the 

opercular  gill.     The  opercular  gill 

is    a    secondary    and    ectodermal 

structure  developed  on   the   inner 

or  posterior  face  of  the  operculum 

(see  below). 

In  the  typical  elasmobranchs  the  interbranchial  septa  extend 

to  the  outside  of  the  body,  and  the  gill  clefts  open  directly  to 

the  exterior,  either  on  the  sides  of  the  neck  (selachii,  Fig.  26) 

or  on  the  ventral  surface  (raiae).      In  the  cyclostomes,  Myxine 

excepted,   there   is  also  a  separate  opening  for  each  gill  cleft. 

In  the  holocephali  a  fold  of  skin  on  either  side  grows  back  over 

the  gill  clefts,  thus  en- 
closing a  space  into 
which  these  empty,  and 
which  in  turn  connects 
with  the  exterior  by  a 
slit-like  gill  opening  be- 


FiG.  24.  Section  through  the 
head  of  a  pig  embryo  6.5  mm.  long, 
showing  the  gill  slits  (1,2,3,)  closed 
by  a  thin  wall.  At  the  left  a  small 
portion  enlarged.  £,  Eustachian 
cleft ;  //,  hyphophysis ;  /)/,  man- 
dibular  cleft ;  P,  pharynx. 


FlG.  25.     Tadpole  of  frog,  showing 
the  external  gill  opening. 


hind.  In  the  ganoids 
and  teleosts  the  same  relations  occur ;  but  in  these  the  fold, 
known  as  the  operculum,  has  a  cartilaginous  or  bony  internal 
skeleton.  In  the  amphibia  the  opercular  fold  is  also  found,  but 


24       MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


in  urodeles  and  caecilians  it  develops  but  slightly.  In  the  anu- 
ran  tadpole,  on  the  other  hand,  the  opercular  folds  of  the  two 
sides  unite  beneath  the  throat,  thus  connecting  the  extra  bran- 
chial chambers  of  the  two  sides,  and  then  the  folds  unite  to  the 
sides  of  the  body,  usually  leaving  but  a  single  opening  on  the 
left  side  through  which  water  is  discharged  from  both  right 
and  left  gills1  (Fig.  25). 

In  the  cyclostomes  the  gill  slits  are  narrow  tubes  widened 
in  the  middle  into  a  saccular  shape  (whence  the  name  marsipo- 
branchs,  pouched  gills,  often  given  the  group).  In  these  sacs 


A  B 

FIG.  26.     Relations  of  gill  clefts,  etc.,  in  an  elasmobranch,  A,  and  a  teleost  B. 

are  the  demibranchs.  In  the  elasmobranchs  the  septa  extend 
to  the  external  surface,  the  gills  not  extending  so  far.  In 
ganoids  and  teleosts,  on  the  other  hand,  the  septa  are  reduced 
to  small  rods  while  the  demibranchs  are  greatly  enlarged. 

In  the  embryonic  amphibia  external  gills  occur.  These 
are  ectodermal  structures  developed  from  the  outer  surface  of 
the  gill  septa2  even  before  the  gill  clefts  break  through.  In 

1  No  operculum  is  developed  in  the  amniotes ;  but  there  is  some  plausibility  in  the 
view  which  regards  the  external  ear  of  the  mammals  as  a  derivation  of  the  ichthyopsidan 
operculum. 

2  Relations,  blood  supply,  and  nerves  go  to  show  that  the  fleshy  processes  (so  called 
balancers)  of  the  urodele  larvae  are  the  modified  external   gills  of   the  hyoid  arch  (see 
Fig.  199). 


RESPIRATORY  ORGANS.  2$ 

the  perennibranch  urodeles  these  external  gills  persist  through 
life.1     In  the  other  urodeles  they  are  lost  without  replacement. 


FIG.  27.     Head  of  young  Amphitima  showing  the  external  gills',  partially  covered 
at  the  base  by  the  backward  extension  of  opercular  fold. 

In  the  anura,  on  the  other  hand,  the  external  gills  are  early 
lost,  and  are  replaced  by  internal  gills  upon  the  sides  of  the 
clefts,  which,  however,  are  said  to  be  of  ectodermal  origin. 

Air-bladder.  —  From  the 
pharyngeal  or  cesophageal 
region  there  arises  also  in 
most  ganoids  and  teleosts 
the  air-  or  swim-bladder.  It 
starts  as  a  diverticulum  from 
the  dorsal  2  wall  of  the  phar- 
ynx, the  distal  portion  of 
which  enlarges  into  a  thin 
walled  sac,  the  air-bladder  or 
pneumatocyst ;  the  proximal 
portion  forms  the  pneumatic 

duct.         This     dllCt     remains  FIG.   28.     Relations  of  the  air-bladder 

open  throughout  life   in  the     to  the  alimentary  canal>  after  Dean-    A> 
ganoids   and   the  lower  tele- 
osts,  but  in  the  higher  tele-     and  dipnoans. 
osts  it  closes  and  is  reduced 

to  a  fibrous  cord.3  The  bladder  itself  usually  lies  dorsal  to 
the  aorta  and  urogenital  system  next  the  vertebral  column.  In 

1  Cope,  however,  claims  that  in  Siren  the  embryonic  gills  are  lost,  and  that  the  per- 
sistent gills  of  the  adult  are  new  structures. 

2  The  pneumatic  duct  empties  laterally  in  some  characinidae,  ventrally  in  Polypterus* 
into  the  oesophagus;  but   until  the  development   is  known,  we  cannot  say  how  far  this 
condition  is  secondary. 

3  The  teleostei  were  formerly  subdivided  into  physostomi,  with  permanent  pneumatic 
duct,  and  physoclisti  with  it  closed. 


in  most  physostomous  teleosts;  B,  in  Ery- 
thrimis;   C,  in  Polypterus,  Calamoichthys9 


26        MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 

shape  it  varies  greatly ;  it  may  be  unpaired,  or  it  may  consist 
of  two  paired  lobes.  It  may  be  a  simple  sac,  or  it  may  be 
subdivided  into  two  or  several  successive  chambers.  Its  in- 
ternal walls  are  usually  smooth,  but  they  may  be  considerably 
convoluted,  thus  greatly  increasing  the  surface.  Occasionally 
its  walls  are  calcified.  Its  chief  function  is  that  of  a  hydro- 
static apparatus.  It  is  not  respiratory,  as  it  receives  arterial 
blood  and  returns  venous  blood.  In  some  fishes  (ostariophysiae) 
it  is  connected  with  the  ear  by  a  Weberian  apparatus,  consisting 
of  a  chain  of  small  bones.  According  to  the  latest  conclusions 
this  apparatus  seems  to  be  for  a  recognition  of  variations  in 
hydrostatic  pressure.  The  swim-bladder  is  absent  in  some  bot- 
tom-living fishes  (pleuronectids,  etc.). 

In  the  pharyngeal  region  of  the  elasmobranchs  are  caeca, 
which  may  be  the  structures  from  which  the  swim-bladder  has 
developed.  The  swim-bladder,  in  turn,  is  usually  regarded  as 
having  given  rise,  by  substitution  of  functions,  to  the  lungs. 
On  the  other  hand,  there  are  some  who  regard  the  lungs  as  new 
formations  in  the  air-breathing  vertebrates,  and  as  having  arisen 
i>y  modification  of  a  pair  of  gill  pouches  which  have  grown 
backwards  instead  of  outwards,  and  consequently  have  failed  to 
form  connection  with  the  ectoderm.  The  method  of  origin  of 
the  lungs  and  the  relations  of  the  cartilages  of  the  larynx, 
shortly  to  be  described,  favor  the  latter  view. 

Lungs.  —  The  lungs  arise  as  an  outgrowth  from  the  ventral 
wall  of  the  pharynx,  just  posterior  to  the  last  gill  pouch.  The 
outgrowth  almost  immediately  divides  into  right  and  left  halves, 
which  grow  back,  laterally  to  the  heart,  into  the  anterior  part 
of  the  body  cavity,  and  the  distal  portions  enlarge  into  thin 
walled  sacs,  the  lungs  proper.  The  proximal  portions  of  the 
paired  outgrowths  form  the  bronchi,  while  the  unpaired  por- 
tion gives  rise  to  the  windpipe  or  trachea,  the  opening  by 
which  the  trachea  communicates  with  the  pharynx  being  the 
glottis. 

In  this  backward  growth  there  is  added  to  the  entodermal 
epithelium  of  these  organs  mesenchyme  tissue,  while  the  lungs, 
invading  the  coelom,  become  covered  externally  with  a  thin 
layer  of  epithelium  (peritoneum,  see  coelom).  Between  the 


RESPIRATORY  ORGANS.  2/ 

two  epithelia  run  numerous  blood-vessels,  —  arteries,  veins,  and 
capillaries,  —  conveying  blood  to  and  from  the  lungs. 

In  the  lower  amphibia  the  lungs  develop  scarcely  beyond 
the  condition  of  simple  sacs  with  respiratory  ducts.1  In  other 
forms,  however,  there  is  increase  of  surface  by  a  folding  of  the 
internal  wall,  to  be  described  later ;  and  in  those  still  higher 
there  is  a  division  of  the  primary  bronchi  into  bronchi  of  sec- 
ondary and  tertiary  orders,  each  of  which  connects  with  a 
separate  division  of  the  pulmonary  organ. 


FIG.  29.  Alimentary  tract  of  human  embryo,  A  at  four  weeks,  B  at  five 
weeks,  after  His.  a,  allantois  stalk  ;  l>,  bile  duct;  c,  caecum;  <?,  epiglottis;  /£,  kid- 
ney; /,  lung;  Im,  lower  jaw;  /,  pancreas;  r,  Rathke's  pocket;  J,  stomach;  sfl, 
Seessel's  pocket;  /,  thymus;  tgt  tongue;  w,  Wolffian  duct. 

In  the  dipnoi  the  trachea  and  bronchi  are  without  skeletal 
supports  in  their  walls  ;  but  in  all  other  forms  cartilaginous  parts' 
are  present,  which  tend  to  keep  the  tube  from  collapse.  In  the 
lower  air-breathers  these  consist  of  separate  pieces  of  cartilage 
on  either  side  of  the  trachea  ;  but  from  the  reptiles  upwards 
they  consist  of  rings  of  cartilage,  incomplete  in  mammals,  the 
gap  on  the  dorsal  surface  of  such  being  crossed  by  membrane 
so  as  to  allow  the  passage  of  food  through  the  overlying 

1  A  considerable  number  of  salamanders  have  recently  been  shown  to  be  lungless ;  even 
the  trachea  has  disappeared,  and  respiration  is  carried  on  by  the  skin. 


28        MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


oesophagus.  The  jointing  of  this  tracheal  skeleton  permits  of 
flexibility.  In  the  bronchi  there  occur  only  irregular  cartilagi- 
nous elements,  which  never  form  rings  or  semi-rings  like  those 
of  the  trachea.  As  a  rule,  the  trachea  pursues  a  straight  course ; 
but  in  certain  birds  (swans,  cranes,  birds  of  paradise,  etc.)  it 
becomes  extensively  convoluted,  its  windings  being  either  be- 
tween the  sternum  and  the  breast  muscles,  or  within  the  breast- 
bone itself. 

At  its  upper  end  the  trachea  becomes  widened  and  special- 
ized, and  is  known  as  the  larynx,  which,  like  the  trachea,  has 

a  cartilaginous  frame- 
work. In  the  lower 
amphibia  this  support 
consists  of  a  pair  of 
cartilages,  the  aryte- 
noid cartilages,  one  on 
either  side,  to  which  are 
added  in  the  higher 
amphibia  a  ring  carti- 
lage, the  cricoid,1  which 
reappears  in  a  similar 
shape  in  the  reptiles. 
In  the  birds  the  larynx 
is  somewhat  rudimen- 

FIG.   30.      Dorsal  (A}  and  ventral  (j?)  views  •«.  i  i     • 

.,  "   i.     ..        tary,    its    place    being 

of  human  laryngeal  apparatus.     A,  arytenoid  carti-  J 

lagej    AH,  anterior  horn  of  thyroid;    C,  cricoid  taken  by  the  Syrinx  to 

cartilage;   £,  epiglottis ;    6" C,  greater  (posterior)  be      mentioned     below. 

horn  of  hyoid;  //hyoid;   LC,  lesser  (anterior)  JR    the     mammals      be_ 

horn  of  hyoid;  Z,  ligament  connecting  hyoid  and  .  . 

thyroid;    PH,    posterior    horn    of    thyroid;     T,  SldCS     the     cnCOld    and 

thyroid  cartilage.  arytenoid s,  there  is 

added  as  a  development 

an  incomplete  ring  of  cartilage  farther  in  front,  the  thyroid  car- 
tilage. This  arises  for  the  most  part  from  the  third  of  the 
visceral  arches,  the  fourth  contributing  to  a  considerable  extent. 
Other  and  smaller  cartilages  are  also  added  in  the  same  group, 
but  need  no  description  here. 

1  This  may  be  the  product  of  fusion  of  the  cartilages  of  the  fifth  gill  arch,  a  view  which 
receives  support  from  the  fact  that  the  muscles  of  the  larynx  are  innervated  by  the  hypo- 
glossal  nerve. 


RESPIRATORY  ORGANS. 


29 


Inside  the  larynx  are  the  vocal  cords.  These  are  folds  of 
the  inner  lining  which  stretch  in  pairs  between  the  thyroid  and 
the  arytenoids,  and,  by  the  motion  of  these  cartilages,  can  be 
tightened  or  relaxed.  The  upper  pair  of  these  are  the  false  vocal 
cords,  so  called  since  they  play  no  part  in  the  production  of  the 
voice.  The  second  or  lower  pair  are  the  vocal  cords  proper. 
Between  the  two  is  a  depression,  the  ventricle  of  the  larynx,  or 
sinus  of  Morgagni,  which  in  certain  apes  becomes  greatly  devel- 
oped as  a  vocal  sac  or  resonator.  In  many  anura  the  floor  of  the 
mouth  is  capable  of  distention,  and  here  serves  as  a  resonator. 


FIG.  31.  Diagram  of  the  relationships  of  the 
visceral  arches  in  man,  after  Wiedersheim.  «,  aryte- 
noid  ;  M,  basihyal ;  c,  cricoid  ;  h,  hyoid  arch  ;  /,  incus ; 
m,  malleus;  me,  Meckel's  cartilage;  J,  stapes;  sp, 
styloid  process ;  /,  thyroid  cartilage ;  /-///  branchial 
cartilages. 


FIG.  32.  Syrinx  of 
Steatornis,  after 
Stejneger. 


In  the  mammals  the  anterior  part  of  the  larynx  becomes 
closely  connected  with  the  hyoid  arch  (see  skeleton). 

In  the  birds,  as  was  said  above,  the  larynx  tends  to  become 
rudimentary.  Its  place  as  a  vocal  organ  is  taken  by  a  '  lower 
larynx,'  the  syrinx,1  developed  at  the  lower  end  of  the  trachea, 
or  at  the  upper  end  of  the  bronchi  where  these  arise  from  the 

1  Rudimentary  in  ostriches  and  some  buzzards. 


3O        MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRATES. 


trachea.  Here  the  cartilage  rings  are  modified  and  coalesced 
into  a  tympanic  chamber,  inside  of  which  are  vibratile  membranes 
which  take  the  place  of  vocal  cords,  while  muscles  running  from 
trachea  to  bronchi  alter  the  tension  of  the  tympanic  walls. 

The  lungs  are  all  but  universally  paired.  In  Ceratodus  the 
single  sac  is  without  trace  of  separation  into  halves  ;  while  in 
some  elongate  vertebrates  (snakes,  caecilians)  one  lung  is  very 
small,  the  other  attaining  great  development. 

The  lungs  in  the  lower  amphibia  are  but  simple  sacs  with 
smooth  internal  walls  ;  but  in  the  frogs  the  internal  surface  be- 
comes folded  so  that  a  number  of  cham- 
bers, the  infundibula,  are  formed,  the  walls 
of  which  in  turn  are  thrown  into  a  number 
of  hemispherical  cups  (alveoli)  lined  with 
pavement  epithelium.  In  the  walls  of  the 
alveoli  run  capillary  blood-vessels.  The  in- 
fundibula open  into  a  large  central  space 
connected  with  the  bronchus,  and  which 
may  be  compared  directly  with  the  bron- 
chioles (infra)  of  higher  forms. 

In    many   reptiles  the   conditions    are 
Dia  ram  of     mucn  as  m  tne  amphibia,   some  (snakes) 
having  the  distal  portion  of  the  lung  with- 
the  margin  are  shown  the     out  infundibular  sacs,  others  having  these 

infundibula,  the  walls  of  complications  of  the  surface  extending 
which  are  lolaed  into 

alveoli.  throughout  the  organ.    In  others  the  bron- 

chus divides  into  secondary  bronchi  as  it 

enters  the  lungs,  each  of  which  is  continued  as  a  bronchiole ; 
or  we  may  have  several  bronchioles  radiating  from  a  single 
bronchus  (Alligator,  Heloderma).  In  some  forms  {Chameleon) 
the  bronchioles  may  connect  with  each  other  distally,  a  matter 
of  interest  in  connection  with  the  parabronchi  of  the  birds 
(infra). 

The  mammalian  lung  may  be  regarded  as  a  complex  of  lungs 
like  those  of  a  frog.  The  primary  bronchus  runs  through  the 
lung,  giving  off  on  either  side  secondary  bronchi,  which  in  turn 
bear  tertiary  bronchi.  Each  of  these  latter  connects  with  small 
tubes,  the  bronchioles,  which  lead  to  infundibula,  as  in  the  am- 


FIG.  33. 

lung    of    frog. 


RESPIRATORY  ORGANS. 


phibia.  These  bronchioles  may  be  as  large  as,  or  even  larger 
than,  the  smaller  bronchi ;  but  they  differ  from  them  in  the  ab- 
sence of  cartilage  and  glands  in  the  wall,  in  the  absence  (usually) 
of  cilia  on  the  internal  surface,  and  in  the  existence  of  alveoli 
arising  directly  from  their  walls.  Besides  the  lobulation  implied 
by  this  branching  of  the  respira- 
tory ducts,  the  lungs  may  also 
be  divided  into  lobes,  varying 
in  number,  clearly  recognizable 
from  the  exterior. 

The  lungs  of  birds  are  pe- 
culiar in  several  respects.  The 
primary  bronchus,  after  entering 
the  lung,  continues  along  the 
ventral  surface  to  near  the  end 
of  the  organ.  Near  its  entrance 
it  gives  off  several  lateral  bron- 
chi, which  also  course  along  the 
ventral  surface,  and  extend  onto 
its  sides.  The  primary  bron- 
chus also  gives  off  from  its  dor- 
sal surface  a  larger  number  of 
secondary  bronchi,  which  extend 
through  to  the  dorsal  surface. 
From  these  dorsal  and  lateral 
bronchi  arise  numbers  of  slen- 
der tubes,  the  so-called  lung- 
pipes  or  parabronchi,  which  are  to  be  regarded  as  modified  bron- 
chioles, since  they  have  similar  walls,  and  since  they  connect 
with  the  infundibula.  They  differ,  however,  from  the  bron- 
chioles of  the  mammals  in  that  they  unite  or  anastomose  fre- 
quently with  each  other,  thus  converting  the  whole  lung  into  a 
network  of  tubes  (compare  the  condition  in  chameleon,  above). 

In  the  embryonic  avian  lung  thin-walled  sacs  arise  from  the 
outer  surface  of  the  lung.  These  air  sacs  increase  in  size,  and  ex- 
tend themselves  in  every  direction,  —  into  the  abdominal  cavity, 
where  they  enter  between  the  viscera,  in  between  the  muscles, 
and  beneath  the  skin  ;  they  enter  many  of  the  bones  (especially 


FIG.  34.  Diagram  of  lung  struc- 
ture in  man.  £,  bronchi ;  BLt 
bronchioles ;  A,  alveolar  duct ;  /, 
infundibulum,  surrounded  by  alveoli. 
Only  a  very  few  bronchi  shown. 


32         MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRATES. 


the  humerus,  femur,  sternum,  and  pelvis),  so  that  the  whole  body 
is  penetrated  by  these  cavities.  While  this  great  extension  of  air 
sacs  is  especially  characteristic  of  birds,  it  has  its  forerunners  in 

the  reptiles,  where,  as  in  the  chame- 
leons, similar  air  sacs  invade  the  ab- 
dominal region,  while  in  the  fossil 
dinosaurs  some  of  the  bones  contain 
cavities  which  are  regarded  as  having 
been  occupied  by  similar  air  sacs.1 
The  function  of  these  air  sacs  in  the 
birds  is  not  certainly  known.  The 
chief  suggestions  made  are  that  they 
largely  increase  the  respiratory  sur- 
face, and  by  introducing  air  into  close 
connection  with  the  tissues,  they  les- 
sen the  demands  upon  the  circulatory 
system  ;  they  also  to  a  slight  extent 
lessen  the  specific  gravity  of  the  ani- 
mal ;  and  it  may  be  that  compression 
of  them  in  one  region  or  another 

^ causes  a  shifting  of   the   position  of 

FIG.  35.    Diagram  of  air-sacs     the  centre  of  gravity,  a  matter  of  no 
of  bird,  the  lungs  shaded.    A      liu]e  importance  in  flight.     Another 

axillary    sac ;     AB,    abdominal         ... 

sac;    AI,   anterior  intermediate      view  IS  that  they  allow  the  air  to  flow 

sac ;  B,  bronchus ;  PI,  posterior     twice   over    the    respiratory    surface, 

intermediate  sac;  PB,  prebron-      thus    allowing    a    more    complete    CX- 

chial  sac ;  SB,  subbronchial  sac ; 

T,  trachea.  change  °f   SaseS' 

Thyroid     Gland. — The      thyroid 

gland  arises  from  the  floor  of  the  pharynx  in  the  neighborhood 
of  the  anterior  gill  slits.  In  the  typical  condition  there  is  a 
median  or  unpaired  invagination  of  the  oral  epithelium,  which 
later  becomes  cut  off  as  a  hollow  vesicle  or  a  solid  body. 
Farther  back,  in  most  if  not  in  all  vertebrates,  a  pair  of  sec- 
ondary invaginations  are  formed.2  Like  the  anterior  invagina- 

1  A  somewhat  similar  pnettmaticity  of  certain  bones  is  found  in  mammals,  and  espe- 
cially in  monotremes,  where  air  cavities  occur  in  certain  bones,  especially  those  of  the  skull. 
These  cavities,  however,  are  not  connected  with  the  lungs. 

2  In  reptiles  but  one  of  these  paired  structures  comes  to  full  development,  the  other 
being  rudimentary. 


RESPIRATORY  ORGANS. 


life 

33 


tion,  these  also  separate  from  the  parent  epithelium,  and  become 
joined  to  the  anterior  portion  in  mammals,  but  retain  their 
distinctness  in  the  lower  forms.  The  thyroid  takes  first  the 
shape  of  numerous  cylindrical  cords  with  internal  lumen.  The 
cords  branch  and  anastomose,  and  blood-vessels 
and  connective  tissue  enter  the  network  thus 
formed. 

By  most  students  the  unpaired  portion  of 
the  thyroid  gland  is  considered  as  a  derivative 
of  the  hypobranchial  groove  of  the  tunicates 
and  AmpJiioxus,  while  the  paired  portion  is 
probably  to  be  regarded  as  derived  from  an  ad- 
ditional pair  of  gill  slits  which  never  break 
through .  to  the  exterior.  The  function  of  the 
thyroid  is  apparently  to  form  some  compound 
of  iodine  necessary  to  keep  the  system  in  good 
condition. 

Thymus  Gland.  —  Closely  connected  with 
the  gill  clefts  in  development  are  a  pair  of 
structures  of  problematical  functions,  —  the 
thymus  glands.  Each  arises  from  the  epithelial 
tissue  on  the  dorsal  margin  of  one  or  more 
gill  slits.  Later  this  tissue  becomes  invaded 
by  leucocytes,  while  ingrowths  of  connective 
tissue  divide  it  up  into  small  lobules.  In  the 
fishes  the  glands  remain  near  these  points  of 
origin  just  above  the  gill  slits  ;  in  the  amphibia 
they  occur  above  and  behind  the  angle  of  the  dersheim. 
jaw.  In  the  reptiles  the  glands  occur  in  the 
neck,  sometimes  far  anterior,  sometimes,  as  in 
the  snakes,  just  in  front  of  the  heart.  In  the 
birds  these  glands  extend  nearly  the  whole 
length  of  the  neck,  while  in  the  mammals  they  pass  backwards 
into  the  thoracic  region  immediately  behind  the  sternum,  and 
ventral  to  the  heart.1  In  mammals  the  thymus  glands  undergo 
more  or  less  complete  degeneration  with  age  ;  in  man  they 
reach  their  highest  development  in  the  second  year. 

1  The  thymus  glands  of  calves  are  the  'throat  sweetbreads'  of  the  market. 


FIG.  36.      Rela- 
tions   of    thyroid 
and   thymus 


£,  epi- 
glottis;   H,    hyoid 

^™*  J*'  *rachea; 

TV,  thyroid  carti- 

lage< 


34        MORPHOLOGY  OF   THE   ORGANS   OF    VERTEBRATES. 

Digestive  Tract.  —  The  alimentary  tract  proper  begins  be- 
hind the  pharyngeal  region  and  extends  to  the  vent.  In  Pe- 
tromyzon,  of  the  cyclostomes,  a  growth  from  the  floor  of  the 
hinder  portion  of  the  pharyngeal  region  extends  forward  above 
the  gill  slits,  so  that  separate  respiratory  and  digestive  tubes 
occur  in  this  region.  In  other  vertebrates  there  is  no  such 
separation.  In  the  cyclostomes  the  alimentary  tract  shows  but 
slight  differentiation  into  regions,  the  point  of  entrance  of  the 
liver  duct  serving  to  divide  it  into  pre-  and  post-hepatic  por- 
tions. In  the  latter  division  a  slight  fold  of  the  internal  sur- 
face forms  a  rudimentary  spiral  valve  recalling  that  to  be 
described  below  in  elasmobranchs  and  ganoids.  In  the  holo-- 
cephali,  some  teleosts,  and  the  lower  urodeles  there  is  scarcely 
more  differentiation  of  the  digestive  canal. 

In  all  other  forms  the  digestive  canal  is  more  or  less  clearly 
divided  into  regions.  Thus  we  find  the  pre-hepatic  portion  dif- 
ferentiated into  an  anterior  slender  tube,  the  gullet  or  oesophagus, 
and  a  posterior  widened  portion  with  glandular  walls,  the  stom- 
ach. The  oesophagus  calls  for  few  remarks.  Its  length  is  cor- 
related with  that  of  the  neck,  and  only  in  certain  birds  is  any 
marked  differentiation  in  its  walls  to  be  seen.  Here  it  becomes 
widened  near  its  middle  into  a  glandular  sac  of  variable  form, 
the  ingluvies  or  crop,  which  serves  as  a  reservoir  of  food,  and 
in  the  pigeons  furnishes  a  food  for  the  young. 

The  stomach,  on  the  other  hand,  presents  numerous  modifi- 
cations. Behind  it  is  usually  sharply  marked  off  from  the  rest 
of  the  alimentary  tract  by  an  internal  fold,  and  by  a  well-devel- 
oped sphincter  muscle  in  its  walls.  This  forms  the  pylorus. 
The  opposite  end  of  the  stomach  is  the  cardiac  region,  so  called 
since  in  man  it  lies  nearest  the  heart.  The  stomach  may  be 
parallel  with  the  axis  of  the  body,  but  usually,  as  in  most  fishes, 
it  is  loop-like,  or  comes  to  lie  more  or  less  at  right  angles  to  that 
axis,  conditions  brought  about  by  a  lengthening  of  the  tract 
more  rapidly  than  the  body  increases  in  length.  Correlated 
with  the  absence  of  teeth,  the  stomach  of  the  bird  acquires  a 
great  development,  and  becomes  divided  into  two  chambers,  — an 
anterior  glandular  portion,  the  proventriculus,  and  a  posterior 
muscular  portion,  commonly  known  as  the  gizzard.  When  most 


DIGESTIVE    TRACT. 


35 


developed,  as  in  grain-eating  birds,  the  muscles  of  this  gizzard 
develop  a  tendinous  disk  on  either  side,  while  the  inner  surface 
is  frequently  lined  with  a  firm  horny  coat  which  aids  greatly  in 
grinding  the  food. 

In  the  mammals  the  line  of  division  between  stomach  and 
oesophagus  is  more  sharply  drawn  than 
elsewhere  in  the  vertebrates.  In  the  seals 
alone  is  the  stomach  parallel  to  the  body 
axis ;  elsewhere  it  is  twisted  into  a  trans- 
verse position.  In  the  mammals  it  also 
shows  greater  variations  of  form  than  in 
any  other  group,  modifications  doubtless 
to  be  explained  by  differences  in  food.  It 
may  be  either  a  simple  sac,  or  it  may  be 
partially  subdivided  into  chambers.  In  the 
simpler  forms  we  may  distinguish  regions 
in  the  stomach,  the  cardiac  and  pyloric 
already  mentioned,  and  between  them  a 
fundus  region  characterized  by  difference 
in  the  glands  lining  the  walls.  When  the 
subdivision  occurs,  the  chambers  corre- 
spond more  or  less  closely  to  these  glan- 
dular regions.  This  division  reaches  its 
extreme  in  the  ruminants,  where  usually 
four  divisions  are  recognized.  These  are 
in  order  (Fig.  38),  (i)  rumen  (paunch), 
(2)  reticulum  (honeycomb),  (3)  omasum  FJG  ^  Digestive 
or  psalterium  (manyplies),  and  (4)  abom-  tract  Of  a  bird  of  prey. 
asum  (rennet).  In  the  cetacea  there  are  <,  crop;  ?,  intestine; 
a  number  of  diverticula  about  the  pyloric  "'•  ,mufc"lar  ston!ach; 

r-7  /,  glandular  stomach;  /, 

region.       It    must,    however,    be    kept    in     trachea;  v,  vent. 
mind   that   the  rumen   and   reticulum   are 

not  truly  gastric  but  oesophageal  in  nature,  and  that  they  serve 
not  as  digestive  organs,  but  for  the  storage  of  food. 

In  the  lower  forms  the  liver  duct  opens  close  behind  the  pylo- 
rus, but  in  the  higher  vertebrates  a  tract  of  some  length,  the 
duodenum,  may  intervene  between  the  two.  While  usually  con- 
sidered as  a  part  of  the  intestinal  region,  this  duodenum  is  really 


30        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

pre-hepatic.  The  post-hepatic  portion  of  the  alimentary  canal  is 
more  or  less  clearly  divided  into  two  regions,  an  anterior  mid 
gut,  the  small  intestine,  and  a  posterior  hind  gut,  the  large 
intestine  of  higher  forms.  In  the  lower  vertebrates  this  distinc- 
tion is  not  so  sharp,  being  largely  indicated  by  the  character  of 
the  internal  walls,  or,  as  in  the  elasmobranchs,  by  the  develop- 


FIG.  38.     Diagram  of   ruminant   stomach,  after  Wiedersheim.     A,  abomasum; 
JP,  psalterium  (manyplies);  RT,  reticulum  (honeycomb);  RU,  rumen  (paunch). 


ment  of  a  caecal  tube  (rectal  gland  or  digitiform  appendix)  at 
the  boundary  between  the  two.  From  the  amphibia  upwards 
the  line  of  division  is  more  sharp,  an  internal  constriction,  the 
ileocolic  valve,  forming  the  line  of  demarcation. 

The  mid  gut  is  the  chief  seat  of  intestinal  absorption,  and 
various  means  are  introduced  of  increasing  the  intestinal  surface. 
In  the  cyclostomes  there  is  an  infolding  of  the  inner  wall  which 
follows  a  slightly  spiral  course.  In  the  elasmobranchs  this  spiral 
valve  acquires  great  development,  either  growing  out  so  that 
the  interior  of  the  intestine  resembles  a  spiral  staircase,  or  more 
like  a  roll  of  paper,  the  free  edge  projecting  into  the  lumen  of 
the  tube.  This  spiral  valve  reappears  in  the  ganoids,  but  is  not 


DIGESTIVE    TRACT. 


37 


/HV 


SP 


:SV 


FIG.  39.  Alimentary  tract  of  dog- 
fish (Acanthias}.  a,  dorsal  aorta  ;  b, 
bile  duct ;  /},  heart ;  z,  intestine,  the 
spiral  valve  showing  through  ;  /,  liver  ; 
m,  mesonphros ;  /,  pancreas  ;  r,  rectal 
gland  ;  s,  spleen  ;  st,  stomach. 


FlG.  40.  Alimentary  canal  of  Pro- 
topterus,  after  Parker.  C,  cloacal 
caecum  ;  G,  gall  bladder  ;  HV,  hepatic 
vein ;  Z,  liver ;  O,  oesophagus ;  OD, 
oviduct;  PV,  pyloric  valve;  R,  rec- 
tum; RD,  renal  (mesonephric)  duct; 
S,  stomach;  SP,  spleen;  -SF,  spiral 
valve ;  F,  vent. 


38        MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRATES. 


found  higher  in  the  scale.1     In  the  higher  fishes  it  is  replaced  by 
caecal  tubes  (pyloric  appendages)  developed  close  to  the  pylorus. 

The    number    of    these 

varies  from  one  in  cer- 

tain ganoids  to  over  one 

hundred  and  fifty  in  the 

mackerel. 

In    the    amphibia 

and  reptiles   the  mid 

gut  is  nearly  straight 

in  the  elongate  forms, 

more    convoluted     in 

the  shorter  types,  the 

convolutions  increasing  in  extent  in  the  birds 
and  mammals.     In  the  birds,  at  about  the  mid-        FlG   42 
die,  the  mid  gut  bears  a  blind  tube,  the  vitel-    ach  and  pyioric  cseca 
line  caecum,  the  remains  of  the  yolk  stalk  of    of  Saimo,  after 

11  i  i  •    T       •       .1  i  •  R&thke.      /,    intes- 

development,  by   which,  in  the  earlier  stages, 


FIG.  41.  Ichthyosauran 
coprolites,  one  in  section. 
The  spiral  character  is  taken 
as  evidence  of  the  presence 
of  a  spiral  valve  in  these 
reptiles.  After  Leunis. 


tine.  ^>        l 

the  intestine  was  connected  with  the  yolk.      In    cseca;  s,  stomach. 
these  higher  forms  increase  of  intestinal   sur- 
face is  brought  about  in  part  by  the  lengthening  of  the  intes- 
tine, and  in  part  by  the  development  of  numerous  small  folds 

(valvulae  conniventes)  and 
minute  finger-like  projec- 
tions (villi)  resembling  the 
pile  of  velvet. 

The  hind  gut  is  hardly 
distinct  in  fishes,  as  viewed 
externally,  but  from  the 
amphibia  on  it  acquires 
greater  individuality.  It 
may  consist  merely  of  a 
straight  tube,  rectum,  or  it 

may  have  a  terminal  rectum  connected  with  the  mid  gut  by  a 
more  or  less  convoluted  tube,  —  the  colon.  Just  behind  the  ileo- 
colic  valve  in  the  forms  from  the  turtles  upwards  is  developed  a 


FIG.  43.  Part  of  small  intestine  of  man, 
showing  the  valvulse  conniventes,  from 
Martin. 


1  The  marks  on  certain  reptilian  coprolites  indicate  that  some  extinct  reptiles  may  have 
had  a  spiral  valve  (Fig.  41). 


DIGESTIVE    TRACT.  39 

blind  tube,  the  intestinal  caecum,1  which  is  clearly  connected  with 
increase  of  digestive  surface.  In  certain  birds  there  may  be  two 
of  these  caeca,  and  in  the  ostrich  there  is  developed  on  its  inside 
a  spiral  fold.  In  the  mammals  the  caecum  shows  great  varia- 
tions. It  is  lacking  entirely  in  certain  groups  (edentates,  chei- 
roptera, some  carnivores).  In  the  herbivorous  forms,  on  the 
other  hand,  it  may  equal  the  body  in  length.  In  man  and  some, 
apes  and  rodents  all  parts  of  the  caecum  are  not  equally  devel- 
oped, the  terminal  portion,  known  as  the  appendix  vermiformis, 
remaining  smaller  than  the  rest. 

In  the  elasmobranchs,  dipnoans,  amphibians,  sauropsida,  and 
the  monotremes  among  the  mammals,  the  rectum  does  not 
open  directly  to  the  exterior,  but  into  a  terminal  enlargement, 
the  cloaca,  into  which  the  urinary  and  reproductive  ducts  also 
empty  ;  and  from  this  chamber  all  contents  pass  to  the  exterior 
through  the  vent.  In  the  other  vertebrates  no  cloaca  is  formed. 
In  connection  with  the  cloaca  in  birds  is  developed  a  sac 
(bursa  Fabricii),  which  comes  to  lie  in  the  pelvic  cavity  be- 
tween the  vertebrae  and  the  terminal  portion  of  the  hind  gut. 
Its  function  is  unknown.  The  bursa  is  said  to  be  of  ectodermal 
origin. 

The  alimentary  tract  is  here  placed  among  the  entodermal 
structures,  but  only  the  lining  coat  is  derived  from  that  germ 
layer.  Other  constituent  parts  are  derived  from  the  mesen- 
chyme.  Beneath  the  entodermal  epithelium  and  following 
closely  its  contour  is  a  layer  of  loose  connective  tissue,  the 
sub-mucosa,2  which  carries  blood  and  lymph  vessels.  Outside 
of  this  are  the  muscular  layers,  two  in  number,  an  inner  circular 
and  an  outer  longitudinal,  each  of  smooth  or  non-voluntary 
muscle.  These  by  their  action  produce  peristaltic  movements 
of  the  contents  of  the  tube.  Where  it  passes  through  the 
body  cavity  the  alimentary  canal  receives  a  third  or  peritoneal 
layer  of  pavement  epithelium  derived  directly  from  the  splanch- 
nic layer  of  the  ccelom. 

1  The  rectal  gland  of  the  elasmobranchs  is  possibly  home  logous  with  the  caecum  of 
the  amniotes. 

2  Occasionally  the  sub-mucosa  may  be  divided  by  a  muscular  layer,  in  which  case  that 
portion  nearest  the  entodermal  epithelium  is  called  the  tunica  propria,  the  deeper  portion 
retaining  the  name  sub-mucosa. 


40        MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRATES. 

Liver.  —  The  liver,  as  was  said  above,  develops  as  a  divertic- 
ulum  from  the  ventral  side  of  the  primitive  alimentary  canal. 
This  outgrowth  branches  again  and  again,  the  result  being  a 
greatly  branched  tubular  gland,  the  proximal  portion  of  the 
tubes  being  specialized  as  the  ducts  leading  to  the  intestine. 
In  the  amphibia  and  reptiles  this  tubular  condition  is  retained 
throughout  life,  the  minute  lumen  of  the  glandular  portions 
being  known  as  the  gall  capillaries.  In  birds  and  mammals 
the  tubular  condition  soon  disappears,  the  gall  capillaries  run- 
ning, without  much  regularity,  between  the  cells.  By  the  in- 


A 


C 


FIG.  44.  Hepatic  ducts  of  A,  frog;  B,  emeu;  and  C,  cat.  A,  ampulla  of 
Vater;  B,  gall-bladder;  C,  cystic  duct;  Z>C,  ductus  choledochus;  H,  hepatic  ducts; 
HE,  hepatoenteric  duct;  W,  duct  of  Wirsung  (pancreatic). 

growth  of  connective  tissue  the  liver  glands  are  divided  into 
lobules,  the  so-called  « liver  islands.'  In  this  connective  tissue 
run  the  larger  gall  ducts  (which  connect  with  the  gall  capil- 
laries), and  also  branches  of  the  hepatic  artery  and  of  the  por- 
tal vein  (see  circulation).  From  their  position  these  vessels 
are  often  spoken  of  as  interlobular.  In  the  centre  of  each 
island  (intralobular  in  position)  is  a  branch  of  the  hepatic  vein, 
while  capillaries  extend  through  the  lobules  from  the  inter- 
lobular to  the  intralobular  blood-vessels.  As  a  rule,  there  is 
but  a  single  duct  emptying  from  the  liver  into  the  intestine, 
and  this,  as  a  rule,  has  connected  with  it  by  a  lateral  branch 
(cystic  duct)  a  thin-walled  gall-bladder.  When  these  condi- 
tions occur,  the  duct  leading  from  the  liver  as  far  as  the  mouth 


DIGESTIVE    TRACT.  41 

of  the  cystic  duct  is  called  the  hepatic  duct;  from  that  point 
to  the  intestines,  the  ductus  choledochus.  Again,  besides 
these  ducts  there  may  also  be  a  separate  hepatoenteric  duct 
leading  directly  from  the  liver  to  the  intestine,  as  well  as  other 
modifications  not  necessary  to  mention,  aside  from  the  numer- 
ous ducts  in  lizards  and  snakes. 

The  liver  thus  formed  is  a  large  compact  organ,  largest  in 
the  lower  vertebrates,  and  larger  in  flesh  (fat)  eating  forms 
than  in  the  herbivorous  groups.  In  many  fishes  it  forms  a 
single,  undivided  mass,  but  in  the  great  majority  of  vertebrates 
two  lobes  are  present,  and  these  in  turn  may  be  lobulated.  The 
blood-vessels  leading  to  the  liver  (portal  vein,  hepatic  artery) 
enter  in  close  relations  to  the  gall  ducts,  while  the  veins 
(hepatic)  leaving  it  are  widely  separate  from  these,  in  contrast 
to  the  conditions  occurring  in  most  organs.  The  liver  is  sup- 
ported by  a  mesentery  (gastro-hepatic  omentum)  which  connects 
it  to  the  ventral  wall  of  the  alimentary  tract,  and  which  is  fre- 
quently continued  below  as  the  suspensory  ligament  of  the  liver. 

The  pancreas  develops  in  much  the  same  way  as  the  liver,  — 
as  an  outgrowth  from  the  entodermal  walls  just  behind  the  liver 
outgrowth.  There  is  the  same  increase  in  size,  while  branching 
gives  rise  to  glandular  portions  and  ducts.  The  pancreas  has- 
recently  been  found  to  occur  in  several  vertebrates  where  its 
existence  was  formerly  denied,  and  farther  research  may  reveal 
one  in  the  cyclostomes  where  none  has  yet  been  found.  Thus 
in  certain  teleosts  its  condition  as  a  delicate  tube  lying  in  the 
mesentery,  and  its  position  in  the  dipnoi  just  outside  the  muscu- 
lar walls  of  the  alimentary  canal,  caused  it  to  be  overlooked  for 
a  long  time.  In  the  elasmobranchs  and  other  teleosts  it  is  a 
well-marked  gland.  In  other  forms  it  is  more  complex  in  its 
origin.  Thus  in  the  ganoids  (sturgeon)  it  arises  by  two  dorsal 
and  two  ventral  outgrowths  ;  in  the  amphibia  and  all  higher 
forms,  from  one  dorsal  and  two  ventral  outpushings,  these  later 
uniting  into  one  glandular  mass.  The  ducts  can  undergo  vari- 
ous modifications,  all  persisting,  or  either  dorsal  or  ventral  dis- 
appearing ;  or  finally  the  ducts  may  come  into  connection  with 
those  leading  from  the  liver  (Fig.  44  C, 


42        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

Two  other  structures,  the  spleen  and  the  urinary  bladder, 
are  closely  connected  with  the  entoderm  in  origin,  but  they  are 
better  described  in  connection  with  those  structures  —  circu- 
atory  and  excretory  —  with  which  they  become  associated  in 
later  life. 


NERVOUS  SYSTEM. 


43 


ECTODERMAL    STRUCTURES. 


THE  derivatives  of  the  ectoderm  may  be  subdivided  into 
epidejmal,  nervous,  and  sensojy  organs,  the  differentiation  of 
the  nervous  from  the  epidermal  structures  beginning  with  or 
even  before  the  completion  of  the  process  of  gastrulation. 


THE    CENTRAL    NERVOUS    SYSTEM. 

The  central  nervous  system  begins  its  development  as  a 
structure  distinct  from  the  rest  of  the  ectoderm  by  the  forma- 
tion of  a  neural  or  medullary  plate  on  the  dorsal  surface  of  the 
embryo.  On  either  side  of  the  primitive  groove  (fused  blasto- 
poral  lips,  p.  7)  the  ectodermal  cells  become  elongated  (cylin- 
drical or  fusiform),  while  in  those  regions  destined  to  give  rise 
to  the  epidermis  they  retain  their  more  flattened  character,  the 
line  between  the  two  regions  being  sharply  drawn.  Soon  after 
being  outlined  the  lateral  edges  of  the  medullary  plate  begin  to 
bend  upwards  and 
inwards,  the 
whole  thus  form- 
ing a  medullary 
groove  bounded 
by  the  medullary 

fnlHQ       rhp      niitpr 
portion      of      each 

fold  being  formed 

by  unaltered  ectoderm  (Fig.  45).  This  inward  bending  of  the 
medullary  folds  continues  until  the  edges  meet,  the  medullary 
plate  being  converted  by  this  process  into  the  walls  of  a  tube, 
which  later  develops  into  brain  and  spinal  cord.  The  edges  of 
the  fold  now  fuse,  —  neural  parts  with  neural,  epidermal  with 
epidermal,  —  so  that  the  nervous  portion  becomes  internal,  and 


FlG.  45.  Section  through  embryo  Acanthias  before 
the  closure  of  the  medullary  groove,  g.  c,  ccelom;  <?, 
entoderm ;  ec,  ectoderm;  ///,  mesothelium ;  ;/,  notochord. 


44        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

is  covered  by  a  continuous  sheet  of  epidermis.  This  process  of 
infolding  and  fusion  proceeds  from  in  front  backwards,  but  at 
the  very  front  end  a  small  opening  (anterior  neuropore)  may 
persist  for  some  time.  The  important  points  to  be  kept  in 
mind  in  this  connection  are  that  the  central  nervous  system 
develops  by  the  infolding  l  of  a  part  of  the  primitively  external 
surface  of  the  body,  and  that  the  inner  surface  of  the  neural 
tube  is  morphologically  external  in  origin. 

Before  the  infolding  of  the  neural  tube  is  completed,  its 
anterior  end  enlarges,  the  first  step  in  the  process  of  differen- 
tiation into  brain  and  spinal  cord.  The  latter  division  must  be 
described  first,  since,  it  presents  much  simpler  conditions  than 
does  the  brain. 

The  spinal  cord  frequently  retains,  to  a  certain  extent,  its 
tubular  character  throughout  life,  although  the  central  canal 
does  not  materially  increase  its  primitive  diameter.  In  the 
earlier  stages  the  cord  is  oval  in  section,  its  sides  being  thick- 
ened ;  while  in  the  median  line,  above  and  below,  it  is  much 
thinner  (Fig.  48).  These  halves  rapidly  increase  in  size  while 
the  central  median  portion  lags  behind,  the  result  being  that 
the  cord  soon  becomes  marked  along  its  ventral  surface  by  a 
longitudinal  groove.  Later  a  corresponding  cleft  appears  on 
the  dorsal  surface.  These  are  the  anterior  and  posterior  fissures 
of  human  anatomy. 

In  sections  of  the  adult  cord  one  clearly  distinguishes  an 
outer  white  matter  and  an  inner  gray  substance ;  the  latter 
takes  the  shape  of  the  letter  H,  the  ends  of  the  uprights  being 
called  the  horns  or  cornua,2  while  the  cross-bar  is  produced  by 
fibres  running  from  one  half  to  the  other  between  the  bottoms 
of  the  fissures  and  the  central  canal.  The  horns  extend 
towards  the  surface,  above  and  below,  thus  dividing  the  white 
matter  of  each  half  of  the  cord  into  three  columns,  —  dorsal,  lat- 
eral, and  ventral ;  the  lateral  column  being  between  the  two 
horns,  the  dorsal  and  ventral  between  the  horns  and  the 

•  l  In  some  forms  (e.g.,  teleosts,  marsipobranchs,  some  ganoids)  the  development  of  the 
central  nervous  system  varies  considerably  from  that  outlined  above,  but  the  final  result  is 
the  same. 

2  A  lateral  horn  (Fig.  46,  cornu  lat.)  must  also  be  recognized  on  the  grounds  of  physi- 
ology and  nerve  origin. 


NERVOUS  SYSTEM. 


45 


fissures.     In  the  later  studies  each  of  these  columns  has  been 
subdivided  (Fig.  46). 

During  the  increase  in  size  of  the  cord,  the  cells  produce  pro- 
toplasmic outgrowths  (axis  cylinders,  p.  n),  some  of  which  run 
forwards  and  backwards  in  the  ventral  and  lateral  columns, 
while  others  pass  outwards  from  the  cord  as  the  motor  roots  of 


Dorsal 
Root 


FIG.  46.  Diagrammatic  section  of  spinal  cord,  after  von  Lenhossek.  Ant. 
pyr.,  anterior  pyramid  tract;  apr,  anteroposterior  reflex  fibre;  c,  collaterals  enter- 
ing the  gray  substance ;  dp,  collateral  of  the  lateral  pyramidal  tract;  GC,  Golgi's 
commissural  cell;  GP,  Golgi's  cell  of  posterior  horn;  Let,  lateral  cerebellar  tract; 
111,  motor  cell  of  anterior  horn ;  ptc,  posterior  tract  cell. 

the  spinal  nerves,  to  be  described  a  moment  later.      From  this 
it   will  be   seen   that   the  white   matter  is  composed  of  nerve 
fibres,  the  gray  matter  of  nerve  cells.      Later  the  dendrites  — 
association  fibres  —  are  formed,  while  with  increase  in  size  blood- 
vessels and  supporting-tissue  press  into  the  cord. 

In  its  early  stages  the  spinal  cord  shows  a  marked  segmen- 
tation or  repetition  of  parts  one  after  the  other.  This  metamer- 


46        MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 

ism  consists  in  alternate  expansions  and  contractions  of  the 
cord  and  its  contained  canal.1  This  early  segmentation  disap- 
pears with  growth,  but  the  same  segments  are  later  indicated  by 
the  roots  of  the  spinal  nerves. 

The  spinal  nerves  are  paired  structures  passing  off  from 
either  side  of  the  cord  (Fig.  47),  each  nerve  arising  by  two 
roots,  one  dorsal  the  other  ventral  in  position.  These  roots 
differ  markedly  in  structure  and  function.  The  dorsal  roots 
are  connected  with  the  dorsal  horn  of  the  gray  matter,  and 
soon  after  leaving  the  cord  each  becomes  enlarged  into  a 
ganglion  composed  of  those  ganglion  cells  which  give  rise  to 
the  fibres  of  which  this  root  is  composed.  The  ventral  roots. 


FIG.  47.  Section  through  spinal  cord  showing  the  roots  of  a  spinal  nerve. 
</,  dorsal  root  with  its  ganglion ;  g,  gray  matter  of  cord ;  v,  ventral  root ;  w,  white 
matter. 

on  the  other  hand,  are  not  ganglionated,  but  their  fibres  are 
connected  with  the  ganglion  cells  of  the  gray  matter  of  the 
ventral  horns  of  the  cord,  from  which  they  pass  out  into  the 
root.  Just  beyond  the  ganglion  of  the  dorsal  root  the  two 
roots  of  a  spinal  nerve  unite,  and  the  fibres  of  each  follow  a 
common  course. 

Experiment  shows  that  the  dorsal  roots  are  sensory ;  i.e., 
they  carry  impulses  from  the  terminal  sensory  structures  to  the 
central  nervous  system.  The  ventral  roots  are  motor  in  func- 
tion ;  that  is,  the  nervous  impulses  which  they  transmit  come 
from  the  central  nervous  system,  and  are  carried  to  peripheral 
portions  (muscles,  glands,  etc.)  which  they  cause  to  act. 
Since  the  dorsal  roots  convey  stimuli  from  without  to  the 

1  It  must  be  understood  that  this  metamerism  is  not  necessarily  primitive  in  its 
character. 


NERVOUS  SYSTEM.  47 

central  nervous  system,  they  are  often  spoken  of  as  afferent 
roots,  while  for  analogous  reasons  the  ventral  roots  are 
termed  efferent.1 

These  roots  differ  also  in  their  mode  of  development.  Cer- 
tain features  of  the  origin  of  the  dorsal  root  are  still  in  dispute, 
but  the  following  statements  are  pretty  generally  accepted.  At 
the  time  of  closure  of  the  neural  tube  a  thin  sheet  of  cells  is 
visible  on  either  side  of  the  line  of  closure  between  the  epider- 
mis and  the  tube.  By  un- 
equal growth  this  sheet  of 
cells,  or  neural  crest,  becomes 
converted  into  segments, 
each  segment  developing  into 
a  ganglion  of  a  dorsal  root. 
Apparently  fibres  grow  out 
from  this  ganglion  to  enter 
the  cord,  while  others  grow  _„  "7T^  ^. 

FIG.  48.     Diagram  of  embryonic  spinal 

peripherally  to  connect  with  cord  with  neural  crest,  c. 

the  sense  organs.      No  such 

crest  is  formed  for  the  ventral  root ;  but  the  fibres  forming  this 
are  connected  with  the  ganglion  cells  of  the  cord,  and  they 
lengthen  with  the  growth  of  the  animal,  so  as  to  connect  with 
the  muscles,  glands,  etc. 

Each  spinal  nerve  soon  divides  into  two  chief  branches,  — 
a  ramus  dorsalis  supplying  the  dorsal  region,  and  a  ramus  ven- 
tralis  distributed  upon  the  sides  and  ventral  surface.  This  latter 
also  gives  off  a  ramus  intestinalis  to  the  viscera.  These  latter 
connect  with  the  sympathetic  system,  a  pair  of  longitudinal 
nerve  cords  with  ganglia  (derived  from  the  spinal  ganglia)  lying 
near  the  junction  of  the  mesentery  with  the  dorsal  wall  of  the 
ccelom.  This  system  supplies  the  digestive  tract,  the  vascular 
system  and  many  glands,  and  in  certain  ichthyopsida  it  may 
extend  into  the  head. 

Typically  the  spinal  nerves  follow  the  myocommata  or  septa 
between  the  muscle  plates  (to  be  described  later),  but  in  all 
forms  above  fishes  in  the  region  of  the  limbs  several  of  the  ven- 

1  Later  studies  show  that  we  must  distinguish  visceral  and  somatic  motor  fibres  ;  vis- 
ceral and  somatic  sensory  tracts.  (See  the  section  on  cranial  nerves.) 


48        MORPHOLOGY  OF   THE   ORGANS  OF   VERJ^EBRATES. 


tral  rami  interlace  to  form  a  plexus  (cervical  and  brachial  in 
front,  lumbar  and  sacral  for  the  hind  limb),  from  which  nerves 
are  distributed  to  the  appendage.1     The  spinal 
cord  is  enlarged  where  the  spinal  nerves  forming 
these  plexuses  are  given  off. 

In  the  early  stages  the  spinal  cord  is 
as  long  as  the  region  of  the  body  sup- 
plied by  it,  but  with  increase  in  size  the 
other  tissues  grow  faster  than  the  cord, 
As  a  result,  the  more  posterior  spinal 
nerves  take  a  very  oblique  course,  while 
the  hinder  end  of  the  cord  is  drawn  out 
into  a  very  slender  thread,  the  filum 
terminale.  The  large  bundle  of  nerves 
which  consequently  extends  be- 
hind the  cord  forms  what  is 
known  as  a  cauda  equina. 
The  brain  is  an  enlarged 
and  immensely 
complicated  ante- 
rior end  of  the 
central  nervous 
system,  and  yet 
we  can  trace  in 
it  some  of  the 
constituents  we 
have  come  to  rec- 
ognize  in  the 
spinal  cord.  Very 
soon  after  the  clo- 
sure of  the  neural 
tube  the  region 
which  is  to  form 
the  brain  becomes 
differentiated  into  three  hollow  enlargements  or  vesicles,  which 
have  received  the  names,  according  to  position,  of  fore,  mid, 

1  A  brachial  plexus  is  formed  in  snakes  and  footless  lizards.     None  is  found  in  the 
caecilians.     Siren  lacks  a  sacral  plexus. 


HI 


FIG.  49.  Right  human  cervical  and  brachial  plex- 
uses (from  Martin)  showing  the  interlacing  of  nerve 
trunks.  C,  I-  VII,  roots  of  cervical  nerves ;  D,  I-III, 
three  anterior  dorsal  roots;  1-4,  nerves  of  cervical 
plexus;  4',  phrenic  nerve  (to  diaphragm)  ;  r,  circumflex  ; 
r,  musculo-cutaneus ;  ic,  internal  cutaneus;  m,  median; 
2,  intercostals;  u,  ulnar. 


NERVOUS  SYSTEM. 


49 


and  hind  brains.  That  these  three  regions  are  not  exactly  com- 
parable to  the  segments  of  the  spinal  cord  is  shown  by  the  fact 
that  the  same  neuromeres  characteristic  of  the  cord  (p.  46) 
appear  in  the  mid  and  hind  brains.  Each  of  these  vesicles 
contains  an  enlarged  portion  (called  primary  ventricle)  of  that 
cavity,  which  in  the  cord  is  called  the  central  canal. 


B 


FIG.  50.  Diagrams  of  the  devel- 
opment of  the  brain.  In  A  the  three 
primary  vesicles;  in  B  the  differentia- 
tion of  the  definitive  regions.  C, 
cerebrum ;  CB,  cerebellum ;  F,  fore 
brain;  H^  hmcl  brain;  Z,  lamina 
terminalis ;  Af,  mid  brain ;  MO, 
medulla  oblongata ;  S,  spinal  cord. 


FIG.  5 1 .  Section  through 
the  brain  of  embryo  pig,  6.5 
mm.  long,  showing  the  seg- 
mentation (neuromeres)  of 
the  hind  brain  («).  o,  otic 
capsule ;  7  &  8,  facial  and 
auditory  nerve ;  9,  glosso- 
pharyngeal  nerve. 


Soon  these  three  vesicles  become  differentiated  by  unequal 
growth  into  five  regions,  the  fore  and  hind  brain  each  giving 
rise  to  two,  the  mid  brain  remaining  unaltered. 

From  the  fore  brain  arise  the  prosencephalon  (telencephalon, 
cerebrum  or  cerebral  hemispheres)  and  the  thalamencephalon 
(optic  thalami,  diericephalon  or  twixt  brain)  in  the  following 
manner.  The  extreme  tip  of  the  fore  brain  in  the  median  plane 
remains  stationary,  and  forms  a  thin  membrane,  later  known  as 


5O        MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 

the  lamina  terminalis.  On  either  side  of  this  the  fore  brain 
grows  outwards,  and  especially  forwards,  thus  producing  two 
lobes,  right  and  left.  These  are  the  cerebral  hemispheres,  while 
the  rest  of  the  primitive  fore  brain  is  the  thalamencephalon. 
The  ventricle  of  the  primitive  fore  brain  participates  in  this  out- 
growth, giving  rise  to  a  cavity  in  each  of  the  lobes ;  so  that  now 
we  have  three  ventricles  in  the  fore-brain  region,  the  first  and 
second  forming  a  pair,  while  the  third  unpaired  ventricle  remains 
in  the  thalamencephalon.  The  paired  ventricles  remain  in  con- 
nection with  the  third  by  small  openings,  the  foramina  of 
Monro. 


TIG.  52.  Sagittal  section  through  vertebrate  brain,  in  front  passing  through 
a  cerebral  lobe,  c,  cerebrum;  cb,  cerebellum;  A,  infundibular  (hypophysial)  out- 
growth; m,  medulla  oblongata;  o,  olfactory  nerves;  ol,  optic  lobes;  /,  pineal- 
structures;  s,  spinal  cord;  1-4,  ventricles. 

While  this  differentiation  is  taking  place  in  the  fore  brain 
the  mid  brain  (known  under  various  names, — mesencephalon, 
optic  lobes,  corpora  bi-  or  quadrigemina)  remains  almost  statio  i- 
ary,  the  chief  change  being  a  thickening  of  its  walls  so  that 
(except  in  teleosts  where  a  part  remains  as  the  epicoele)  the 
primitive  ventricle  of  its  earlier  condition  becomes  a  narrow 
tube,  the  iter  or  aqueduct,1  connecting  the  third  ventricle  with 
the  ventricle  in  the  hind  brain. 

In  the  hind  brain  the  differentiation  is  largely  confined  to 
the  dorsal  surface.  It  consists  in  the  outgrowth  from  the  an- 
terior dorsal  wall  of  a  lobe  of  tissue  which  extends  backward 
over  the  rest,  and  forms  the  cerebellum  or  metencephalon,  the 
rest  of  the  hind  brain  forming  the  medulla  oblongata  or  myelen- 
cephalon,  which  passes  into  the  spinal  cord  behind. 

The  different  regions  of  these  five  divisions  of  the  brain 
become  variously  developed,  the  walls  being  thickened  in  parts 

1  Iter  e  tcrtio  ad  quartuin  vcntriculum  ;  aqneductus  Sylvii. 


NERVOUS  SYSTEM. 


while  in  others  they  remain  but  a  cell  or  two  in  thickness.  In 
the  cerebral  hemispheres  the  lower  or  ventral  surface  develops 
a  large  ganglionic  mass,  the  corpus  striatum,  in  either  hemi- 


FIG.  53.  Nearly  median  section  of  brain  of  trout,  after  Rabl-Ruckhard.  a, 
aqueduct;  act  anterior  commissure;  b,  bulbus  olfactorius;  c,  ventriculus  communis 
(composed  of  first  three  ventricles  of  typical  brain);  cb,  cerebellum;  cs,  corpus 
striatum  ;  F,  frontal  bone  ;  /*,  habenular  ganglion  ;  hy,  hypophysis  ;  z,  infundibulum  ; 
/,  lobus 'inferior ;  m,  medulla;  /,  pinealis;  /a,  pallium  of  cerebrum ; /<:,  posterior 
commissure ;  s,  saccus  vasculosus ;  /,  torus  longitudinalis  ;  /<?,  tectum  of  optic  lobes ; 
z/,  valvula  cerebelli;  /,  77,  olfactory  and  optic  nerves;  7F,  fourth  ventricle. 

sphere.  The  rest  of  the  cerebral  wall  is  known  as  the  pallium 
or  mantle,  and  undergoes  great  modifications  in  the  different 
groups.  In  some  fishes  (cyclostomes,  ganoids,  and  teleosts) 
it  is  epithelial  in  character.  In  other  vertebrates  it  is  largely 


FIG.  54.     Brain  of  dog  (after  Wiedersheim),  showing  fissures  and  gyri  of 
cerebrum.     II-XII>  cranial  nerves. 

nervous  in  nature,1  its  outer  surface  (cortical  substance)  being 
composed  of  ganglion  cells.  In  all  the  lower  vertebrates  the 
surface  of  the  cerebrum  is  smooth,  but  in  the  higher  mammals 

1  Even  in  mammals  a  portion  —  the  septum  pellucidum  —  retains  an  epithelial  character. 


52        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

(educabilia)  fissures  appear  in  its  surface,  separating  convolu- 
tions or  gyri,  and  the  higher  the  mammal  the  more  numerous 
the  convolutions.  It  will  readily  be  seen  that  this  produces  an 
increase  in  surface,  and  consequently  of  cortical  (ganglionic) 
substance  ;  and  it  is  noteworthy  that  this  increase  is  correlated 
with  increase  of  intelligence. 

From  its  anterior  ventral  region  each  hemisphere  gives  off 
an  olfactory  lobe  (rhinencephalon)  into  which  a  part  of  the  ven- 
tricle may  extend.  Connected  with  each  olfactory  lobe  is  an 
olfactory  ganglion  which  may  be  placed  either  in  the  cerebrum 
itself,  or  may  be  carried  out  towards  the  end  of  the  olfactory 
lobe.  From  these  lobes  arise  the  olfactory  nerves  (see  below). 
In  the  diencephalon  the  lateral  walls  become  thickened  into 
large  tracts,  the  thalami,  while  the  dorsal  wall  as  a  whole  retains 
its  epithelial  character,  becoming  variously  folded  to  form  the 

anterior  choroid  plexus,  which  car- 
ries blood-vessels  into  the  three 
anterior  ventricles.  From  this 
dorsal  surface  are  also  developed 
three  structures,  —  pinealis,  epi- 
physis,  and  paraphysis,  —  to  be 
mentioned  again  in  connection 
with  the  sense  organs.  From 
FIG.  55.  Sagittal  section  through  the  pinealis  a  pair  of  nerve  tracts, 
head  of  larval  Petromyzon,  after  the  habenulae,  run  along  the  inner 

von  Kupffer.      c.  notochord;   h,  in-  •  ,            r    ,,        ,11         •         ^r-i         n 

vagination  for  hypophysis ;  <,  infun-  SldeS    <*    the    thalam1'       TllC    fl°°r 

dibulum;  ;//,  mouth  cavity;   n,  nasal  of      the      thalamencephalon      gives 

involution;/,  pineal  outgrowth.  rjse    to    a    hollow    Outgrowth,     the 

'^infun dibulum,  which  extends 

backwards  and  downwards,  developing  from  its  extremity  tis- 
sue, which  unites  with  other  cells,  derived  directly  from  the 
ectoderm,  to  form  the  hypophysis  or  pituitary  body.  This  ecto- 
dermal  portion  arises  from  the  ectoderm  between  the  nose  and 
the  mouth,  or  from  the  mouth  itself,  and  grows  upwards  and 
inwards  to  join  the  infundibular  portion.  For  a  time  it  retains 
its  connection  with  the  parent  layer  by  means  of  a  cord  of  cells, 
the  hypophysial  duct,  which  later  disappears.  The  significance 
of  these  ventral  structures  of  the  twixt  brain  is  very  obscure. 


NERVOUS  SYSTEM. 


I 


A  plausible  suggestion  is  that  the  infundibulum  represents  the 
invertebrate  mouth,  the  ectodermal  portion  of  the  hypophysis  a 
modified  pair  of  sense  organs.  The  optic  nerves  are  outlined 
as  hollow  outgrowths  from  the  sides  of  the  twixt  brain,  while 
on  its  ventral  surface 
may  be  developed  ac- 
cessory structures, — 
the  lobi  inferiores, 
sacculi  vasculosi,  cor-  i /.•%*.*:•:••.. 

pus    albicans    (corp.  J£fe:k 

mam  mil  are)  tuber 
cinereum,  etc. 

A  topographic 
point  is  to  be  kept  in 
view,  —  the  cerebral 
hemispheres  and  the 
diencephalon  are  in 
front  of  the  anterior 
end  of  the  notochord 
—  are  prechordal. 

The  mesencepha- 
lon  has  its  walls 
thickened  so  that  its 
contained  ventricle,  in  the  higher  groups,  is  reduced  to  the  nar- 
row aqueduct  already  mentioned.  Its  dorsal  surface  is  divided 
by  a  longitudinal  groove  into  right  and  left  lobes  (corpora  bi- 
gemina),  and  these  in  turn  may  each  be  subdivided  transversely 
into  two  (corpora  quadrigemina).1  Leading  ventrally  and  for- 
wards from  these  lobes  in  all  except  the  cyclostomes  are  the 
optic  tracts  connecting  with  the  optic  nerves.  The  floor  of  the 
mid  brain  is  formed  by  a  pair  of  fibre  tracts — crura  cerebri  — 
separated  by  a  longitudinal  fissure. 

The  cerebellum  or  metencephalon  is  a  thickening  of  ner- 
vous matter  on  the  dorsal  anterior  end  of  the  hind  brain.  It 
may  exist  as  a  small  transverse  fold,  or  it  may  be  greatly 
enlarged,  extending  forwards  over  part  of  the  mid  brain,  and 
backwards  over  the  anterior  end  of  the  medulla.  It  may  be 

1  In  older  works  the  anterior  of  these  lobes  were  called  nates  ;  the  posterior,  testes. 


FIG.  56.  Two  stages  in  the  development  of  the 
hypophysis  in  the  pig  ;  A  in  an  embryo  10  mm.  long, 
Bin  15.5  mm.  long.  E,  epithelium  of  roof  of  mouth; 
//,  hypophysis  connected  with  the  mouth  cavity  in  A 
by  the  hypophysial  duct,  in  B  by  the  solid  hypo- 
physial  stalk  HS',  /,  infundibulum. 


54        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

unpaired  in  appearance,  or  it  may  consist  of  a  pair  of  lateral 
lobes  or  hemispheres  separated  by  a  median  portion  or  vermis, 
terminating  in  a  small  lobe,  the  valve  of  Vieussens,  which  roofs 
in  the  fourth  ventricle  in  front. 

The  myelencephalon  or  medulla  oblongata  is  the  cranial 
extension  of  the  spinal  cord,  presenting  behind  but  slight  dif- 
ferences from  that  structure.  In  front  it  widens,  while  its  roof 
thins  out  and  becomes  epithelial  and  folded,  to  form  the  poste- 
rior choroid  plexus  for  the  underlying  fourth  ventricle.  This 
region  of  thinning  is  known,  from  its  shape,  as  the  fossa  rhom- 
boidalis.  It  is  bounded  in  front  by  the  valve  of  Vieussens,  and 
on  either  side  by  the  diverging  dorsal  columns  o.f  the  cord 
(p.  44),  which  are  frequently  subdivided  into  a  median  fasciculus 
gracilis  (Column  of  Goll)  and  a  more  lateral  fasciculus  cuneatus 
(Burdach/s  Column).  (Fig.  46.)  Each  dorsal  column  receives 
in  front  fibres  from  the  lateral  column,  the  whole  forming  an 
enlargement,  corpus  restiforme,  on  either  side,  from  which  the 
posterior  peduncles  of  the  cerebellum  pass  forward  and  upward 
into  the  metencephalon.  On  the  ventral  surface  are  the  anterior 
ends  of  the  ventral  columns  (p.  44),  here  known  as  the  pyra- 
mids. These  can  be  followed  forward  until  they  pass  into  the 
crura  cerebri  already  mentioned.  In  the  higher  vertebrates  the 
anterior  ends  of  the  pyramids  are  crossed  by  transverse  bundles, 
forming  the  pons  Varolii,  which  act  as  commissures  connecting 
the  two  halves  of  the  cerebellum.  The  medulla  oblongata  is 
further  noticeable  since  it  gives  rise  to  the  greater  part  of  the 
cranial  nerves. 

The  various  parts  of  the  brain  are  connected  by  longitu- 
dinal fibre  tracts  and  by  transverse  fibres  or  commissures. 
Some  of  these  have  already  been  mentioned,  and  some  others 
may  be  noticed  here.  The  chief  longitudinal  tracts  are  those  of 
the  pyramids,  which  may  be  followed  through  the  crura  cerebri 
to  the  corpus  striatum.  Some  of  the  fibres  of  the  lateral 
column  and  a  part  of  those  of  the  dorsal  column  enter  the 
cerebellum  through  the  posterior  peduncles  of  the  cerebellum, 
while  the  majority  from  these  columns  end  in  the  medulla. 
From  the  cerebellum,  fibres  extend  forward  into  the  mid  brain 
through  two  bands  of  tissue  known  as  the  anterior  peduncles  of 


NERVOUS  SYSTEM. 


55 


the  cerebellum,  which 
enter     the    posterior 
portion   of    the   optic 
lobes.    The  habenulae 
are    also    to     be    re- 
garded   as    longitudi- 
nal tracts  ;  while  the 
for  nix,    a    part    of 
which  lies  ventral  to 
the   corpus    callosum 
(infra),    is    to    be 
placed    in    the    same 
category,   although 
its     fibres     seem     in 
places  to    run   transversely. 
Among    the   transverse 
fibres  most  constant  are  the 
anterior  and  posterior  com- 
missures  in  the    region   of 
the  twixt   brain.1     The  an- 
terior crosses  from  side  to 
side  in  the  anterior  wall  of 
this    region,    the    other    is 
nearer  the  junction  of  twixt 
brain  and    optic   lobes.      In 
the  higher  vertebrates   the 
cerebral   hemi- 

FIG.  57.   Diagram  of  fibre 

spheres     are     con-   tracts  in  mammalian  brain> 

nected    by    a    large    after  Jelgersma.       C,  cortex 

of  cerebrum  ;  CB,  cortex  of  cer- 
ebellum ;  CF,  centrifugal  tract  ;  FO,  centrifugal  tract  to 
olivary  nucleus;  GR,  nucleus  ruber  ;  ND,  dentate  nucleus 
of  cerebellum;  NO,  nucleus  olivarius  inferior;  OC,  crossed 
connective  between  olivary  nucleus  and  the  vermis;  P,  gan- 
glion of  the  pons;  PC,  dorsal  tract  from  the  pontal  ganglion  to 
the  cerebellar  cortex  of  the  opposite  side  ;  PD,  pyramid  tract ; 
PDC,  tract  from  the  pedunculus  cerebelli  to  the  cerebrum; 
RT,  fibre  course  from  nucleus  ruber  to  optic  thalamus;  TC,  connection  of 
with  cerebral  cortex  ;  THO,  optic  thalamus;  VPC,  ventral  tract  from  portal 
to  cerebellar  cortex  of  the  opposite  side. 

1  The  so-called  median  commissure  is  not  a  fibre  tract. 


thalamus 
ganglion 


56        MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES, 

transverse  band,  the  corpus  callosum.  Traces  of  this  occur  in 
amphibia  and  reptiles,  but  it  acquires  its  highest  development 
in  the  higher  mammals.  The  pons  Varolii,  passing  beneath  the 
anterior  pyramids  of  the  cord,  similarly  connects  the  cerebellar 
hemispheres  in  the  higher  vertebrates.  Here,  too,  must  be  num- 
bered the  decussation,  or  crossing  of  the  fibres  of  the  anterior 
pyramids  from  one  side  to  the  other. 


il 

FIG.  58.  Longitudinal  section  of  the  brain  of  a  frog,  after  Gaupp.  The  epi- 
thelium blocked,  ah,  anterior  part  of  hypophysis ;  ca,  anterior  commissure ;  cb, 
cerebellum;  cc,  corpus  callosum;  ch,  optic  chiasma ;  e,  epiphysis ;  fm,  foramen  of 
Monro ;  he,  habenular  commissure ;  i,  iter ;  li,  infundibular  lobe ;  Id,  lamina  ter- 
minalis,  infraneural  portion  ;  Its,  lamina  terminalis,  supraneural  portion ;  o,  olfac- 
tory lobe ;  pc,  posterior  commissure  ;  pci,  inferior  and  median  choroid  plexus ;  pcp> 
posterior  choroid  plexus;  q,  posterior  portion  of  mid  brain;  rn,  recessus  neuropori ; 
ro,  recessus  opticus;  v,  velum  medullare  ant.  ;  y,  4?',  third  and  fourth  ventricles. 

In  its  earlier  stages  the  brain  lies  in  the  same  horizontal 
plane  with  the  spinal  cord.  Soon,  by  unequal  growth  of  its 
dorsal  and  ventral  surfaces,  bends  or  flexures  appear.  Most 
constant  of  these  is  the  cephalic  flexure  between  fore  and  mid 
brains,  by  which  the  axis  of  the  fore  brain  is  bent  ventrally  at 
nearly  right  angles  to  the  rest.  Two  other  flexures  may  also 
appear  ;  they  are  most  prominent  in  mammals.  The  pontal 
flexure,  in  the  region  of  the  pons  Varolii,  is  in  the  opposite 
direction  ;  the  nuchal  flexure,  in  the  medulla,  is  ventral  again. 
In  the  ichthyopsida  these  flexures  large!/  disappear  with  growth  ; 
in  the  amniotes  they  persist  throughout  life. 

In  the  lower  groups  the  five  divisions  of  the  brain  are  sub- 
equal  in  size,  but  the  higher  vertebrates  are  characterized  by 
a  great  increase  in  size  of  the  cerebellar,  and  especially  of  the 
Cerebral,  regions,  so  that  these  completely  cover  over  the  twixt 


NERVOUS  SYSTEM.  57 

and  mid  brains.  The  backward  extension  of  the  cerebrum  is  es- 
pecially marked  in  mammals.  Connected  with  this  overgrowth 
is  the  formation  of  the  fifth  ventricle,  or  pseudo-ventricle,  a 
cavity  in  no  way  connected  with  the  true  ventricles,  but  lying 
morphologically  outside  the  brain,  between  the  septa  pellucida, 
the  fornix,  and  the  corpus  callosum. 

The  brain  and  spinal  cord  are  enclosed  in  envelopes  of 
mesenchymatous  origin,  which  hold  them  in  position,  and  serve 
as  the  bearers  of  nutrient  vessels,  etc.  These  membranes  from 


FIG.  59.  Sagittal  section  through  the  head  of  pig  embryo  of  15.5  mm.  length, 
showing  the  cranial  flexures.  AA,  axis  of  brain;  C\  cephalic  flexure;  //,  hypophy- 
sis ;  7/7',  heart  ;  J\f,  mouth  ;  7',  pontal  flexure  ;  7',  tongue.  The  nervous  tissue  dotted. 

outside  to  inside  are  the  dura  mater  and  the  pia  mater.  Of 
these  the  dura  is  a  more  dense  connective  tissue,  consisting  of 
two  lamellae  in  the  lower  vertebrates  ;  its  blood-vessels  being 
distributed  to  the  walls  of  the  spinal  canal  and  the  skull.  The 
pia  is  more  delicate,  and  bears  the  blood-vessels  of  the  brain  and 
cord.  Between  the  two  layers  is  a  large  lymph  space,  and  in 
the  amphibia  and  higher  vertebrates  this  is  divided  by  a  third 
membrane,  —  the  arachnoid.  The  pia  enters  all  the  fissures 
and  depressions  in  the  brain  and  cord,  carrying  nourishment 
into  the  nervous  mass. 


58        MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRATES. 

Cranial  Nerves.  —  Like  the  cord,  the  brain  gives  rise  to 
nerves,  but  these  nerves  present  many  differences  from  those 
of  the  spinal  region.  The  last  word  concerning  these  has  yet 
to  be  written,  but  the  following  outline  summarizes  our  present 
knowledge,  as  well  as  indicates  some  of  the  directions  in  which 
modifications  of  our  ideas  may  be  expected. 


ncl 


FIG.  60.     Base  of  human  brain  (from  Martin),  showing  roots  of  cranial  nerves, 
I-XII.     ncf,  first  cervical  nerve. 

The  nerves  arising  from  the  brain  (cranial  nerves)  are  in 
pairs,  which  have  received  names  and  numbers  in  man  ;  and 
these  have  bees  transferred  to  the  corresponding  structures  in 
the  lower  vertebrates  as  follows  :  — 

I.    Olfactory. 
II.    Optic. 
III.    Oculomotor. 


NERVOUS  SYSTEM.  59 

IV.  Trochlearis  (or  Patheticus). 

V.  Trigeminal  (or  Trifacial). 

VI.  Abducens. 

VII.  Facial. 

VIII.  Auditory, 

IX.  Glossopharyngeal. 

X.  Vagus  (or  Pneumogastric). 

XI.  Spinal  Accessory  (or  Accessory  of  Willis). 

XII.  Hypoglossal. 


As  has  been  described,  the  spinal  nerves  contain  both  sen- 
sory and  motor  roots.  The  cranial  nerves  present  some  dif- 
ferences from  this.  Thus  nerves  I.,  II.,  and  VIII.  are  purely 


FIG.  61.  Diagram  of  cranial  nerves  (shark),  a,  alveolaris;  b,  buccalis;  c, 
cerebrum;  cl>,  cerebellum;  r/,  chorda  tympani;  e,  ear;  er,  external  rectus  muscle; 
/,  inferior  rectus  muscle ;  g,  Gasserian  ganglion;  h,  hyoid  cartilage;  Am,  hyoman- 
dibular  cartilage  ;  hmd^  hyomandibular  nerve ;  ?',  internal  rectus  muscle ;  ?'<?,  inferior 
oblique  muscle;/',  Jacobson's'commissure ;  /,  lateralis  branch  of  vagus;  m,  mouth; 
Jin ,  Meckel's  cartilage;  md,  mandibularis ;  mx,  maxillaris  superior ;  w,  nose;  o, 
optic  lobes  (mesencephalon)  ;  op,  ophthalmicus  profundus;  os,  ophthalmicus  super  - 
ficialis;  /,  pinealis;  //,  palatine;  f>o,  post-trematic  branch;  /«,  intestinal  (pneu- 
mogastric)  branch  of  vagus  ;  fr,  pre-trematic  branch  ;  //</,  pterygoquadrate  cartilage  ; 
s,  spiracle ;  so,  superior  oblique  muscle ;  5;-,  superior  rectus  muscle ;  /,  thalamen- 
cephalon  ;  I-X,  cranial  nerves;  1—5,  gill  clefts.  f 


sensory ;   III.,  IV.,  and  VI.  are  solely  motor  ;  while  the  others 
are  mixed,  i.e.,  contain  both  motor  and  sensory  fibres. 

Both  the  olfactory  and  the  optic  nerves  are  usually  regarded 
as  differing  from  all  other  cranial  nerves  in  that  they  arise  as 


60        MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 

hollow  outgrowths  of  the  brain  itself.1  The  olfactory  nerve 
arises  from  the  olfactory  lobe,  and  is  distributed  to  the  sensory 
epithelium  of  the  nose.  Like  all  other  sensory  nerves  it  is  pro- 
vided with  its  own  ganglion,  which  may  either  be  included  in 
the  brain,  or  it  may  be  carried  out  into  close  proximity  with 
the  olfactory  organ.  It  is  evident  that  the  two  cases  are  really 
different,  and  that  we  can  only  speak  of  true  olfactory  nerves 


FlG.  62.  Diagram  of  cranial  nerves  in  an  amniote ;  nerves  II  to  IV and  VI 
much  as  in  the  ichthyopsida,  and  hence  omitted.  <7,  accessorius  ;  c,  ciliary  ganglion  ; 
<•/,  chorda  tympani ;  j\  f  acialis ;  _////,  branches  of  facialis  to  facial  muscles;  g>  glosso- 
pharyngeal ;  //,  hypoglossal ;  ?,  ramus  intestinalis  of  vagus  ;  _/',  Jacobson's  commissure  ; 
/,  ramus  lingualis  ;  m,  maxillaris  ;  vict,  mandibularis ;  o,  ophthalmic ;  og;  otic  ganglion; 
/,  palatine;  s,  submaxillary  ganglion;  sp,  sphenopalatine  ganglion;  A  tympanum. 

distal  to  the  olfactory  ganglion.  The  connection  between  the 
olfactory  ganglion  and  the  brain  is  made  by  the  olfactory  tract. 
The  optic  nerves,  which  arise  primitively  from  the  ventral 
sides  of  the  diencephalon,  have  their  ganglia  lying  upon  the 
superficial  portion  of  the  retina  (see  eye,  below).  They  retain 
their  connection  with  the  thalamencephalon-  throughout  life  in 

1  In  connection  with  nerves  I.  and  II.  it  is  to  be  noted  that  the  posterior  cranial  and  the 
spinal  nerves  of  selachians  are  at  first  hollow  outgrowths  from  the  brain  (or  neural  crest). 
Farther,  that  the  definitive  nerve  of  the  adult  grows  back  from  the  ganglion  to  join  the 
brain  in  both.  These  facts  tend  to  invalidate  the  distinction  drawn  between  nerves  I.  and 
II.  and  the  others. 


NERVOUS  SYSTEM.  6 1 

the  cyclostomes,  but  in  the  higher  group  they  become  con- 
nected secondarily  with  the  optic  lobes  by  means  of  the  optic 
tracts.  These  optic  tracts  are  so  formed  that  the  nerves  cross 
beneath  the  thalami,  that  from  the  right  eye  going  to  the  left 


A  B  CD 

FIG.  63.     Diagrams  of  optic  chiasma,  after  Wiedersheim.     A,  most  teleosts; 
J3,  herring;   C,  Lacerta;  D,  higher  mammals. 

optic  lobe,  and  vice  versa.  There  may  be  a  simple  crossing  or 
an  interlacing  of  fibres,  or  a  complete  union  of  the  trunks  (optic 
chiasma). 

Nerves  III.,  IV.,  and  VI.  are  purely  motor  nerves,  supplying 
the  muscles  which  move  the  eye.  The  oculomotor  arises  from 
the  crura  cerebri,  and  supplies  the  muscles  rectus  superior,  in- 
ternus,  inferior,  and  obliquus  inferior.  The  trochlearis  arises  from 
the  posterior  dorsal  portion  of  the  mid  brain,  although  its  centre 
inside  the  brain  lies  ventrally.  It  supplies  the  superior  oblique 
muscle.  The  abducens  arises  from  the  anterior  pyramids,  and  is 
distributed  to  the  externus  rectus  muscle  and  to  the  retractor 
bulbi,  when  this  muscle  is  present.  The  oculomotor  is  always 
distinct,  but  the  others  may  be  fused  with  the  fifth,  and  in 
some  animals  their  existence  has  not  yet  been  demonstrated. 

The  trigeminal  nerve  arises  from  the  anterior  end  of  the 
sides  of  the  medulla.  It  is  always  large,  and  in  the  higher 
vertebrates  at  least  has  two  distinct  roots,1  the  dorsal  root  bear- 
ing a  ganglion  (Gasserian  or  Casserian  ganglion).  As  its  name 
implies,  it  has  three  branches,  —  a,  ophthalmicus  prof undus,  dis- 
tributed chiefly  to  the  nose  and  lachrymal  region  ;  b,  maxillaris 
superior,  supplying  the  region  of  the  upper  jaw ;  and  <:,  the 
mandibularis  or  maxillaris  inferior,  going  to  the  lower  jaw,  and 
in  amniotes  to  the  tongue.  Frequently  the  last  two  are  united 
for  a  distance  as  a  maxillaris  nerve.2  Branches  a  and  b  are 

1  In  at  least  some  of  the  ichthyopsida  these  two  roots  can  be  distinguished  by  micro- 
scopic study,  although  not  by  ordinary  dissection. 

2  The  terminology  of  the  trigeminal  and  facial  used  here  is  believed  best  to  express  the 
relations  of  the  branches. 


62        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

largely  sensory,  most  of  the  motor  fibres,  together  with  sensory, 
going  to  branch  c.  Each  of  these  branches  may  have  a  secon- 
dary ganglion  connected  with  it,  —  the  ciliary  ganglion  on  a, 
the  sphenopalatine  on  b,  and  the  otic  on  c. 

In  many  ichthyopsida  the  seventh  nerve  is  closely  connected 
with  the  fifth,  and  by  mere  dissection-  the  roots  of  the  two  can- 
not be  distinguished.1  In  the  higher  vertebrates  the  two  nerves 
are  distinct  throughout.  The  facialis  is  more  complicated  than 
the  trigeminal,  and  may  contain  four  components.  In  the  lower 
vertebrates  it  is  a  mixed  nerve,  but  in  the  higher  it  is  purely 


vnb- 


m 


FIG.  64.  Diagram  of  the  relations  of  the  fifth  (shaded),  seventh,  and  eighth 
nerves  in  an  aquatic  amphibian,  after  Strong,  b,  buccalis ;  ga,  auditory  ganglion ; 
gg,  Gasserian  ganglion  ;  gp,  palatine  ganglion  ;  h,  hyoid  nerve  ;  m,  mandibular  nerve ; 
mx,  superior  maxillary  nerve  ;  op,  ophthalmicus  profundus;  os,  ophthalmicus  super- 
ficialis ;  /,  palatine  nerve ;  VII  a,  aa,  ab,  the  three  roots  of  the  seventh  nerve  j 
VIII,  root  of  auditory  nerve;  IX,  communication  of  seventh  with  ninth  nerve 
(Jacobson's  commissure). 

motor,  and  is  connected  largely  with  the  muscles  of  expression. 
In  its  greatest  development  (ichthyopsida)  it  gives  rise  to  four 
branches,  —  a,  ophthalmicus  superficialis  ;  '2  b,  hyomandibularis  ; 
c,  buccalis  ;  and  d,  palatinus.  The  first  of  these  has  its  own 
ganglion  and  is  purely  sensory,  supplying  the  lateral  line  organs 
(see  sense  organs,  infra)  on  the  top  of  the  head.  It  is  found 
only  in  aquatic  ichthyopsida,  the  frog,  for  instance,  losing  it  at 

1  Microscopic  study  shows  that  they  are  usually  as  distinct  here  as  in  the  higher  forms. 

2  Fibres  from  the  fifth  accompany  the  ophthalmicus  superficialis. 


NERVOUS  SYSTEM.  63 

the  time  of  metamorphosis.  The  hyomandibularis  soon  divides 
into  an  anterior  or  mandibular  branch  and  a  posterior  division, 
which  supplies  the  muscles  of  the  gill  cover,  and  some  of  those 
of  the  jaw.  When  the  first  visceral  cleft  or  spiracle  is  present, 
this  division  takes  place  just  above  it,  so  that  one  branch  (man- 
dibularis)  is  pre-trematic,  i.e.,  is  in  front  of  the  opening,  the 
other  being  post-trematic  (Fig.  61).  The  mandibularis  goes  to 
the  lower  jaw;  and  one  of  its  branches,  which  unites  with  the 
mandibularis  branch  of  the  fifth  nerve,  is  known  among  the 
higher  vertebrates  as  the  chorda  tympani.  The  palatine  branch 
supplies  the  palate  and  the  roof  of  the  mouth.  In  the  lower 
forms  it  is  a  mixed  nerve  ;  in  the  mammals  it  innervates  only 
the  muscles  of  the  soft  palate.  It  may  unite  with  either  branch, 
a  or  b,  of  the  fifth.  The  buccal  branch  runs  in  the  upper  jaw, 
uniting  with  the  ophthalmicus  profundus. 

The  auditory  nerve  is  closely  connected  with  the  seventh, 
and  is  often  regarded  as  its  dorsal  root.  It  goes  directly  to  the 
ear,  dividing  almost  immediately  into  two  branches,  which  may 
leave  the  skull  through  separate  foramina. 

The  vagus  complex  is  composed  of  the  ninth,  tenth,  and 
eleventh  nerves,  which  are  closely  connected,  and  present  many 
similarities  to  each  other.  In  many  features  they  resemble 
more  closely  the  spinal  nerves,  especially  in  the  presence  of 
distinct  dorsal  and  ventral  roots.  The  ear  intervenes  between 
these  and  the  nerves  in  front.  The  complex  arises  from  the 
side  of  the  medulla  by  from  four  to  eight  or  more  roots,  the 
anterior  pair  being  considered  as  those  of  the  glossopharyngeal. 
Usually  in  the  aquatic  vertebrates  its  ganglion  is  fused  with 
that  of  the  vagus. 

The  glossopharyngeal  nerve  splits  into  two  branches,1  the 
anterior  going  to  the  pharyngeal  region,  the  other  (lingualis) 
to  the  muscles  and  mucous  membrane  of  the  gill  in  fishes,  and 
to  the  sense  organs  of  the  tongue  in  the  mammals,  etc.  The 
pharyngeal  branch  also  gives  off  a  nerve  (Jacobson's  anasto- 
mosis) which  unites  with  the  hyomandibularis  of  the  facial. 

The  vagus  or  pneumogastric  has  a  wide  distribution.      In 

1  In  the  branchiate  vertebrates  the  divisicm.occurs  above  the  first  true  gill  slit,  so  that 
here,  too,  we  have  pre-  and  paglMfrernat  jc  bVandife.  'V  . 

ft      IIKMVTR^ITY     1 


64        MORPHOLOGY  OF  THE   ORGANS   OF   VERTEBRATES. 

aquatic  vertebrates  it  divides  into  two  main  trunks,  a  ramus 
lateralis  (possibly  equivalent  to  the  r.  dorsalis  of  a  spinal 
nerve),  which  is  lacking 'in  the  terrestrial  forms,  and  a  ramus 
intestinalis.  The  lateralis  branch  runs  the  length  of  the  body, 
either  close  beneath  the  skin,  or  deeper  in  the  muscles  near  the 
vertebral  column.  It  is  purely  sensory,  and  is  distributed  to 
the  lateral  line  organs  .of  the  trunk  ;  and  the  absence  of  these 
structures  in  the  amniote  vertebrates  explains  the  disappearance 
of  the  nerve.  The  ramus  intestinalis  is  the  pneumogastric 
nerve  of  human  anatomy.  It  is  largely  motor  (or  better,  in- 
hibitory) in  its  functions.  It  is  distributed  to  pharynx,  stomach 
(air-bladder  of  fishes),  and  the  respiratory  apparatus,  gills  and 
lungs.  Of  the  branches  to  the  gills  there  are  as  many  as  there 
are  gill  clefts  behind  the  one  supplied  by  the  ninth  nerve.  Each 
branch  divides  above  the  gill  cleft  into  pre-  and  post-trematic 
branches. 

The  accessory  of  Willis  is  apparently  a  spinal  nerve  which 
in  the  amniotes  enters  into  close  association  with  the  vagus. 
Its  distribution  is  chiefly  to  the  muscles  connected  with  the 
neck  and  shoulder  girdle,  e.g.,  sternocleidomastoid  and  trapezius. 

The  hypoglossal  nerve  is,  in  the  adult  vertebrate,  purely 
motor,  its  branches  being  distributed  to  the  muscles  of  the 
tongue  and  to  some  of  those  of  the  hyoid  region.  It  is  only  in 
the  amniotes  that  this  can  be  considered  as  a  cranial  nerve ; 
in  the  ichthyopsida  it  does  not  enter  the  skull.  It  is  interest- 
ing to  find  that  in  the  larval  stages  of  some  forms  this  nerve 
has  a  dorsal  ganglionated  root,  while  in  certain  species  two  such 
roots  have  been  found,  a  fact  which  tends  to  show  that  the  nerve 
is  really  compound. 

Within  recent  years  it  has  been  recognized  that  the  compo- 
nents of  the  spinal  and  cranial  nerves  were  more  numerous  than 
is  implied  by  the  account  given  on  pages  46  and  59.  In  the 
spinal  nerve  it  is  clear  that  a  distinction  must  be  made  between 
the  nerves  of  the  body  (somatic  nerves)  and  those  of  the  viscera 
(visceral  nerves).  Each  of  these  is  made  up  of  sensory  and 
motor  parts,  so  that  four  components  are  to  be  recognized:  (i), 
somatic  sensory  (general  cutaneous)  ;  (2),  somatic  motor  ;  (3), 
visceral  sensory ;  and  (4),  visceral  motor.  The  ganglion  cells 


NERVOUS  SYSTEM.  65 

of  the  first  are  situated  in  the  spinal  ganglia,  and  the  nerves 
terminate  in  the  dorsal  horn.  The  ganglion  cells  of  the  somatic 
motor  nerves  lie  in  the  ventral  horn,  and  the  nerves  leave  by  the 
ventral  roots.  The  internal  relations  of  the  visceral  system  are 
not  so  evident  ;  but  both  are  possibly  related  to  the  lateral  horn 
region,  the  visceral  sensory  nerves,  whose  centres  in  the  trunk 
region  are  in  the  sympathetic  ganglia,  entering  by  the  dorsal 
roots,  while  the  visceral  motor  nerves  leave  by  both  dorsal  and 
ventral  roots  (not  proved  for  mammals) 


rr      red 


plv-.- 


FIG.  65.  Diagram  of  the  sensory  components  in  the  cranial  nerves  in  Menidia, 
after  C.  J.  Herrick.  General  cutaneous  component  white;  communis  (visceral) 
dotted;  lateralis  black;  the  outline  of  the  brain  shaded,  b,  gill  clefts;  bg,  branchial 
ganglia  of  the  vagus,  the  last  containing  the  ganglion  of  the  ramus  intestinalis ; 
ct,  pre-trematic  branch  (chorda  tympani)  of  facialis  ;  dl,  dorsal  lateral  line  ganglion 
of  the  facialis  ;  fc,  fasciculus  communis  ;  gg,  Gasserian  ganglion  ;  gn,  geniculate 
ganglion;  /,  general  cutaneous  (jugular)  ganglion  of  the  vagus;  Iv,  lobus  vagi;  w5, 
os1,  ophthalmicus  superficial  of  fifth  and  seventh  nerves;  red,  ramus  cutaneus 
dorsalis  of  vagus ;  rlv,  ramus  lateralis  of  the  vagus ;  rot,  ramus  oticus ;  rp,  ramus 
palatinus  of  the  facialis ;  rr,  ramus  recurrens  of  the  facialis ;  s,  ramus  supratempora- 
lis  of  the  vagus;  spv,  spinal  V  tract  (ascending  root  of  the  trigeminal);  ta,  tuber 
acusticum ;  thm,  hyomandibular  trunk;  tm,  inferior  trunk,  containing  the  rami 
maxillaris  and  mandibularis  of  the  trigeminal,  the  buccalis  of  the  facial  and  com- 
munis fibres;  vl,  ventral  lateral  line  ganglion  of  the  facialis;  /,  olfactory;  //,  optic; 
VIII,  auditory;  IX,  glossopharyngeal. 


66        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

In  the  cranial  region  the  matter  is  still  further  complicated 
by  the  appearance  of  a  lateralis  system,  the  nerves  of  which  are 
distributed  to  the  ear  and  to  the  lateral  line  system,  and  to  no 
other  organs.  In  terrestrial  vertebrates,  where  the  lateral  line 
system  is  lost,  the  lateralis  nerves,  with  the  exception  of  the 
eighth  (auditory),  are  lacking.  The  fibres  of  the  lateralis  com- 
ponents terminate  in  the  tuber  acusticum.  The  relations  of  the 
sensory  components  of  the  cranial  nerves  are  shown  in  Fig.  65, 
in  which,  for  clearness,  the  motor  elements  have  been  omitted. 
The  somatic  motor  nerves  of  the  head  include  only  the  eye- 
muscle  nerves  (III.,  IV.,  VI.).  Visceral  motor  fibres  are  found 
in  the  fifth,  seventh,  ninth,  and  tenth  nerves, 


SENSE    ORGANS. 

All  sensory  organs  of  vertebrates  arise  from  the  ectoderm. 
Some  remain  throughout  life  connected  with  the  surface  of  the 

bodyj^^-^epidermis,  —  while  others 
sink  into  special  structures  for 
their  protection,  the  sense  cap- 
sules. With  few  exceptions  sense 
organs  are  formed  of  specialized 
cells,  —  sense  cells,  —  each  of 
which  is  connected  by  afferent 
nerve  fibres  with  the  central  ner- 
vous system.  Between  the  sense 
cells  there  may  be  other  ecto- 
dermal  cells  which  have  a  sup- 
porting function,  or  which  serve 

FIG.  66. '  Sense  cells,  after  various  & 

authors.    A,  taste  cell  of  rabbit;  B,     to  isolate  the  sensory  cells  from 

hair  cell  from  lagena  of  pigeon;   C,      each    Other. 

These  sense  organs  which  are 
situated  in  the  epidermis  are  the 
more  generalized,  and  among 
them  are  distributed  the  sensations  of  touch,  pressure,  and  tem- 
perature. In  the  aquatic  ichthyopsida  (Fig.  67)  these  organs 
are  composed  of  rod-like,  club-formed,  or  pear-shaped  cells,  the 
free  extremities  of  which  may  reach  the  surface ;  but  in  all 


olfactory  cell  of  Proteus;  D  and  £, 
rod  and  cone  cells  from  the  human 
eye. 


SENSE   ORGANS, 


67 


FIG.  67.  Lateral  line  organ 
of  Atnblystoma,  showing,  be- 
neath, the  nerve  fibres ;  on  the 
free  surface  the  sensory  hairs. 


terrestrial  vertebrates  where  the  surface  of  the  skin  is  dry,  the* 
sensory  structures  sink  to  a  deeper  position. 

Lateral  Line  Organs.  — Some  of  these  organs  are  irregularly 
distributed,  while  others  are  grouped  into  regular  series,  and 
form  what  are  known  as  the  lateral 
line  organs.  In  their  early  stages 
these  lateral  line  organs  are  upon 
the  surface.  Later  they  sink,  in  the 
amphibia,  info  pits,  in  pisces  into  lon- 
gitudinal grooves  which  may  be  closed 
into  tubes,  with  openings  at  regular 
intervals.  With  increase  in  size  of 
the  animal,  the  number  of  openings 
also  increases  by  division.  The  open- 
ings frequently  perforate  scales,  while 

the  canals  between  them  may  become  enclosed  in  bone,  espe- 
cially upon  the  head.  By  the  presence  of  grooves  and  canals 
in  the  skulls  of  many  fossil  forms,  we  infer  that  they  pos- 
sessed lateral  line  organs.  There  is  considerable  variation  in 
the  distribution  of  the  lines  of  these  organs,  but  the  following 

are  the  most  constant 
series:  (i),  the  lateral 
line  of  the  trunk  (may 
be  double)  which  ex- 
tends the  length  of  the 
body  between  the  dorsal 
and  ventral  musculature ; 
this  series  gives  the  name 
to  the  whole  system;  (2), 
occipital  series,  crossing 
the  back  of  the  head  and 
connecting  the  systems 
of  the  two  sides  ;  (3), 
supraorbital,  and  (4),  in- 


FIG.  68.  Dorsal  view  of  head  of  sturgeon, 
showing  the  distribution  of  the  lateral  line  canals, 
after  Collinge.  de,  dermethmoid ;  do,  dermo- 
occipital ;  ee,  dermoectethmoid ;  <?/,  epiotic; 
fr,  frontal;  m,  main  canal;  oc,  occipital  com- 
missure; pa,  parietal;  //,  post-temporal;  so, 
suborbital  canal;  spo,  supraorbital  canal;  sq, 
squamosal. 


fraorbital  series,  running  respectively  above  and  below  the  eye  ; 
(5),  mandibular  series,  upon  the  lower  jaw.  In  these  grooves 
or  canals  are  the  groups  of  sensory  cells,  the  groups  on  the  head 
being  innervated  by  the  ophthalmicus  superficialis,  buccalis  and 


68        MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRATES. 


mandibularis  externus  branches  of  the  seventh  nerve,  those  on 
the  trunk  by  the  lateralis  branch  of  the  tenth  nerve.  These 
organs  occur  in  the  aquatic  stages  of  the  amphibia ;  but  upon 
the  assumption  of  terrestrial  life,  as  in  salamanders,  frogs,  etc., 
the  organs  are  lost  and  their  nerves  disappear. 

In  selachians  and  ganoids  are  found,  especially  on  the  snout, 
other  sense  organs,  known  as  ampullae  and  Savi's  vesicles,  the 
functions  of  which  are  more  problematical  than  even  those  of 
the  lateral  line  organs.  The  ampullae  may  be  organs  of  pres- 
sure sense. 

Allied  to  the  sense  organs  of  the  lateral  line  are  structures 
known  as  end -buds.  These  consist  of  a  number  of  sensory 

cells,  each  bearing  sen- 
sory hairs,  compacted 
into  a  bud-like  mass,  and 
surrounded  by  supporting 
cells.  In  the  cyclostomes 
and  fishes  they  are  scat- 
tered over  the  surface, 
but  from  dipnoi  upwards 
they  are  confined  to  the 


cavities    of    the    mouth 
and    nose,    and     in    the 
higher  vertebrates  to  the 
oral  cavity.     In  the  mam- 
mals, these  function  as  organs  of  taste,  and  the  same  is  prob- 
ably true  of  the  lower  vertebrates,  since  certain  fishes  have  been 
shown  to  be  capable  of  tasting  with  the  external  skin. 

Sense  Corpuscles.  —  In  the  terrestrial  vertebrates  the  epi- 
dermal sense  organs  take  a  great  variety  of  shapes  due  to  the 
modifications  of  the  accessory  structures  —  sometimes  unicellu- 
lar, sometimes  multicellular  in  character  ;  but  in  all  these  we  prob- 
ably have  to  do  with  free  nerve  terminations  on  or  between 
the  accessory  cells.  In  all  cases  these  structures  are  buried  in 
the  deeper  layer  of  the  epidermis  or  in  the  dermis  beneath. 
The  simplest  are  oval  cells,  the  deeper  face  of  each  seated  in 
a  cup-like  expansion  of  a  nerve  termination.  In  the  compound 
tactile  cells  (Grandry's  or  MerkePs  corpuscles),  found  only  in 


FIG.  69.     Taste  buds   (end   buds)   from  the 
human  mouth  (from  Martin). 


SENSE   ORGANS. 


FlG.  70.  Gran- 
dry's  corpuscle, 
after  Bohm  and 
Davidoff.  n,  axis 
cylinder  of  nerve. 


FIG.  71.     Pa- 

cinian  corpuscle. 
;/,  axis  cylinder  of 
nerve. 


birds,  two  or  more  biscuit-shaped 

cells  are  included  in  a  connec- 
tive tissue,  while  the  connecting 

nerve  becomes  flattened  out  into 

disks  between  each  two  cells.    A 

more  complicated  type  is  found 

in    the  corpuscles   of   Vater   or 

Pacini, —  elliptical  structures 

composed  of  layers  of  cells  like 

the  layers  of  an  onion,  into  the 

centre    of    which    projects    the 

axis  cylinder  of  a  sensory  nerve. 

Under  the  heading  of  tactile  cor- 
puscles (Wagner's  or  Meissner's  corpuscles)  are 
included   club-shaped   aggregations  of  cells,  around  which  are 
coiled  the  terminal  nbrillae  of  a  nerve.     These  last  are  scattered 
all  over  the  body  in  the  amphibia,  but  are  more  restricted  in 
their    distribution      in     the     higher 
groups. 

Among  tactile  organs  must  also 
be  enumerated  the  long  facial  hairs 
(vibrissae)  of  mammals,  the  base  of 
each  being  surrounded  by  a  network 
of  nerves.  Besides  these  special 
tactile  organs  there  are  numerous 
free  nerve  terminations  in  the  epi- 
dermis of  all  vertebrates  from  cyclo- 
stomes  to  mammals  to  which  sensory 
functions  must  be  ascribed. 

The  ears  in  all  vertebrates  are 
paired  structures  on  either  side  of 
the  head  between  the  seventh  and 
ninth  nerves.  In  the  most  highly 
developed  ears  three  portions  are  to 
be  distinguished,  —  inner,  middle, 
and  outer,  —  the  first  of  which  only 
is  sensory  and  essential,  and  is  the 
only  part  occurring  in  the  fishes ; 


FlG.  72.  Meissner's  corpuscle 
from  human  finger,  after  Law- 
dowski  from  Wiedersheim.  a, 
fibrous  tissue  envelope;  b,  cor- 
puscle with  its  cells;  «,  entering 
medullated  nerves ;  n' ',  ramifica- 
tions of  nerves  ;  «",  club-shaped 
nerve  terminations. 


7O       MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


the  other  two  are  accessory  in  character.  The  sensory  portion 
of  the  inner  ear  arises  from  the  ectoderm.  At  first  it  is  a  cup- 
like  depression  on  either  side  of  the 
head.  Then  it  sinks  deeper  and  its 
edges  unite,  converting  the  cup  into  a 
closed  sac,1  the  primitive  otic  vesicle. 
In  all  forms  the  closure  lags  at  one 
point,  and  in  this  way,  by  the  in  sinking 
of  the  rest,  a  slender  tube,  ductus  en- 
dolymphaticus  or  aqueductus  vestibuli, 
is  formed,  reaching  to  the  parent  ecto- 
derm ;  and  in  the  elasmobranchs  this 
tube  opens  throughout  life  to  the  ex- 
terior by  a  small  opening  near  the 
middle  line  of  the  top  of  the  head. 
FIG.  73.  Involution  of  audi-  In  other  forms  it  becomes  closed,  and 

tory ^  epithelium,  A,  to  form  the     in    Some    groups  the  ducts    of    the  two 
auditory  vesicle  in  the  embryo     gides  COnnect    above    the    brain, 

tern.    CJ3,  cerebellum ;  G,  audi-  ' 

tory  ganglion ;  A;  notochord.       Distally  each  duct  expands  into  a  sac- 

cus   endolymphaticus,    which    in    the 
lamprey,  according  to  Ayers,  is  sensory. 

The  otic  vesicle  is  at  first  spherical,  or  oval,  but  it  soon  divides 
by  constriction  into  an  upper  portion,  the  utriculus,  and  a  lower, 
sacculus,  connected  by  a  narrower  utriculo-saccular  canal. 
Flattened  outgrowths  arise  from  the  walls  of  the  utriculus,  the 
walls  of  which  become  pinched  together  so  that  each  outgrowth 
becomes  converted  into  a  semicircular  canal,  opening  at  either 
end  into  the  utriculus.  In  the  myxinoids  there  is  but  one  of 
these  canals ;  the  lampreys  have  two,  and  all  other  vertebrates 
three.  Two  are  in  vertical  planes  at  nearly  right  angles  to  each 
other,  and  from  their  position  are  known  as  the  anterior  and 
posterior  canals ;  the  third  is  horizontal  in  position,  and  is 
called  the  external  canal.  Each  canal  bears  an  enlargement, 
the  ampulla,  at  one  end , 2  —  at  the  anterior  end  of  the  horizontal 
canal,  at  the  ventral  ends  of  the  vertical  canals. 

1  In  some  forms  (e.g.,  amphibia)  only  the  deeper  layer  of  the  ectoderm  participates  in 
the  formation  of  the  otic  vesicle.     Otherwise  the  history  is  much  the  same. 

2  The  single  semicircular  canal  of  the  myxinoids  has  an  ampulla  at  either  end. 


SENSE   ORGANS. 


The  sacculus  is  always  con- 
nected with  the  ductus  endolym- 
phaticus,  and  it  gives  off  behind 
an  outpushing  known  in  the  lower 
vertebrates  as  the  lagena.  In  the 
mammals  this  lagena  becomes 
greatly  developed,  and  forms  the 
scala  media  of  the  cochlea  de- 
scribed below. 

As  long  as  the  otic  vesicle  re- 
mains a  simple  sac,  it  bears  on  its 
surface  a  single  patch  of  sensory 
epithelium  ;  but  with  differentiation 
of  parts,  the  epithelium  becomes  cor- 
respondingly divided  into  a  num- 
ber of  maculae  (the  sensory  cells 
of  which  bear  short  sense  hairs) 
and  cristae  (provided  with  long 
hairs).  In  the  lampreys,  where 
there  is  no  sacculus,  there  are  but 
three  of  these  patches,  —  a  crista 
in  each  ampulla  and  a  macula  in 
the  vesicle.  In  other  forms  there 
are  three  cristae,  and  at  least  one 
macula  in  the  utriculus,  two  in  the 
sacculus,  and  one  in  the  lagena. 

These  parts,  derived  from  the 
ectoderm,  form  the  membranous 
labyrinth.  It  is  filled  with  a  fluid, 
the  endolymph,  in  which  are  otoliths 
or  particles  of  calcic  carbonate, 
sometimes  of  microscopic  size,  but 
in  the  teleosts  forming  'ear 
stones'  of  considerable  magnitude. 
The  membranous  labyrinths  are 
protected  by  the  otic  capsules  de- 
scribed in  connection  with  the 
skull.  These  are  laid  down  in 


FIG.  74.  Diagram  of  the  mem- 
branous labyrinth.  A,  anterior 
canal;  A  A,  anterior  ampulla; 
AE,  external  ampulla ;  AP, 
posterior  ampulla :  C,  utriculo- 
saccular  canal;  D,  ductus  endo- 
lymphaticus;  £,  external  (hori- 
zontal) canal ;  P,  posterior  canal ; 
/*,  recessus  utriculi;  S,  saccus 
endolymphaticus;  SA,  sacculus; 
U,  utriculus. 


FlG.  75.  Ear  of  Myjcine,  after 
Retzius.  tf,  ampullar  crista  ;  ap, 
posterior  ampulla  ;  c,  semicircular 
canal;  ca,  anterior  ampulla;  d, 
ductus  endolymphaticus  ;  M,  ma- 
cula; n,  nerves;  s,  sacculus  endo- 
lymphaticus. 


72        MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRATES, 


CO, 


raft 


mit 


rec 


ms 


FIG.  Jrt>.  Membranous  labyrinth  of  thrush 
( Turdus},  after  Retzius,  from  Wiedersheim. 
aa,  anterior  ampulla;  ac,  eighth  nerve;  #/, 
posterior  ampulla;  ca,  anterior  canal;  ce,  ex- 
ternal canal ;  <:/,  posterior  canal ;  de,  ductus  en- 
dolymphaticus  ;  /,  lagena ;  mn,  macula  neglecta ; 
ms,  macula  sacculi;  mu,  macula  utriculi ;  />/, 
papilla  lagense  ;  raa,  nerve  to  anterior  ampulla  ; 
rap,  to  posterior  ampulla ;  rb,  basilar  nerve ; 
reC)  recessus  utriculi;  r/,  nerve  to  lagena;  ;-//, 
nerve  to  macula  neglecta ;  s,  sacculus ;  sc,  septum 
conciatum  ;  sfl,  posterior  utricular  sinus  ;  ss, 
superior  utricular  sinus;  /z/,  tegmentum  vasculo- 
sum ;  u,  utriculus 


FIG.  77.  Section  through  the  cochlea  of  a 
cat.  Bone,  shaded.  C,  organ  of  Corti ;  G,  spiral 
ganglion;  N,  nerve;  SM,  S7\  SV,  scalse 
media,  tympani,  and  vestibuli ;  R,  Reissner's 
membrane. 


cartilage,  but  in  all  except 
the  lower  vertebrates  the 
cartilage  is  finally  replaced 
by  bone.  The  inner  walls 
of  these  capsules  follow 
more  or  less  closely  the 
contour  of  the  membra- 
nous labyrinth,  thus  con- 
stituting  the  skeletal 
labyrinth,  between  which 
and  the  membranous  por- 
tions is  a  space  filled  with 
the  perilymphatic  fluid. 
The  walls  of  these  cap- 
sules are  perforated  in- 
ternally for  the  passage 
of  nerves,  etc.,  while  on 
their  lateral  surfaces,  in 
all  groups  above  amphibia, 
are  two  openings,  —  the 
fenestra  ovalis  and  the 
fenestra  rotunda  (the  lat- 
ter  crossed  by  mem- 
branes), —  through  which 
sound  waves  pass  to  the 
parts  described. 

In  the  mammals  the 
skeletal  labyrinth  follows 
very  closely  the  membra- 
nous portion,  and  in  one 
part  these  structures  need 
a  further  description. 
That  part  called  the  la- 
gena in  the  lower  verte- 
brates is  greatly  devel- 
oped here,  and  is  drawn 
out  and  coiled  in  a  spiral, 
which  is  accompanied, 


SENSE   ORGANS.  73 

above  and  below,  by  similar  outgrowths  of  the  perilymphatic 
space.  From  the  resemblance  which  these  structures  present  to 
a  spiral  stairway  these  divisions  are  called  scalae,  that  part  con- 
nected with  the  membranous  labyrinth  being  the  scala  media, 
the  upper  of  the  perilymphatic  spaces  being  the  scala  vestibuli, 
the  lower  the  scala  tympani.  This  whole  structure,  from  its 
resemblance  to  a  snail-shell,  is  called  the  cochlea.  In  the  scala 
media  the  macula  lagenae  of  the  lower  vertebrates  becomes  de- 
veloped into  a  highly  specialized  sensory  structure,  —  the  organ 
of  Corti.  Besides  '  hair  cells  '  (sensory  cells)  and  other  cells, 
the  organ  consists  of  series  of  hard  rods  (pillar  cells)  arranged 
like  a  A  at  right  angles  to  the  axis  of  the  scala.  As  the  spiral 


qa   POH 


FIG.  78.  Organ  of  Corti  in  section,  after  Stohr.  AT,  auditory  tooth;  CC9 
cells  of  Claudius;  DC,  Deiter's  cells;  HC,  Hensen's  cells;  HP,  head  plate;  Iff, 
inner  hair  cells ;  MB,  basal  membrane  ;  N,  nerve  ;  OH,  outer  hair  cells ;  IP,  OP,  in- 
ner and  outer  pillar  cells ;  /*,  phalanges  ;  SS,  sulcus  spiralis  ;  T,  tunnel.  The  '  mem- 
brana  tectoria,'  being  decidedly  problematic  in  character  and  relations,  omitted. 

diminishes  in  size,  from  apex  to  base  these  A's  also  diminish  in 
size,  a  fact  which  led  to  the  view  formerly  held  that  these  were 
in  some  way  connected  with  the  recognition  of  pitch. 

The  middle  ear  or  tympanum  first  appears  in  the  anura.1 
It  is  formed  by  the  expanded  end  of  the  first  visceral  cleft 
(spiracle  of  elasmobranchs),  which  does  not  break  clear  through 
to  the  exterior,  but  is  closed  externally  by  a  thin  tympanic 
membrane,  with  an  external  wall  of  ectoderm,  an  inner  of  ento- 
derm,  and  a  middle  layer  of  mesenchyme.  Internally  the  tym- 
panic cavity  remains  in  connection  with  the  pharynx  by  means 
of  the  proximal  portion  of  the  cleft,  here  known  as  the  Eusta- 
chian  tube.  Sound  waves  are  conducted  across  the  tympanic 

1  In  the  urodeles  and  caecilians  the  tympanic  cavity  is  lacking,  and  there  is  but  a 
single  auditory  ossicle,  the  stapes,  which  usually  articulates  with  the  quadrate. 


74   MORPHOLOGY  OF  THE  ORGANS  OF  VERTEBRATES.   • 

cavity  by  means  of  auditory  ossicles  which  extend  from  the 
tympanic  membrane  to  the  fenestra  ovalis.  In  the  anura  and 
sauropsida  there  are  two  of  these  ear  bones,  the  stapes,  situ- 
ated in  the  fenestra  ovalis,  and  the  columella,  extending  from 
the  stapes  to  the  tympanic  membrane.  In  the  mammals  the 
columella  is  replaced  bV  two  bones,  the  incus  and  the  malleus, 
neither  of  which  can  be'homologized  with  the  columella. 


FiG.  79-  Diagrammatic  section  of  human  ear,  from  Martin  after  Czermak.  At 
auditory  nerve;  a,  ampulla;  B,  b,  semicircular  canal;  G,  external  meatus;  k,  carti- 
lages; M,  concha;  o,  fenestra  ovalis;  P,  tympanic  cavity  with  chain  of  bones;  /*/, 
scala  tympani,  r,  fenestra  rotunda;  fi,  Eustachian  tube;  S,  cochlea;  Vt,  scala 
vestibuli. 

The  stapes  arises  as  a  chondrincation,  and,  later,  ossifica- 
tion of  the  membrane  closing  the  fenestra  ovalis;  the  columella 
is  post-spiracular,  and  may  in  part  correspond  to  the  hyoman- 
dibular ;  the  incus  is  apparently  the  quadrate  of  the  lower  ver- 
tebrates ;  while  the  malleus  is  the  proximal  end  of  Meckel's 
cartilage  (?  os  articulare)  which  becomes  cut  off  from  the  rest.1 

In  all  anura  and  in  many  reptiles  the  tympanic  membrane  is 
on  the  outer  surface  of  the  body,  but  in  higher  groups  the  mem- 

1  There  is  great  uncertainty  upon  some  of  these  points,  different  students  having 
different  ideas  of  the  homologies.  The  view  given  here  is  based  upon  personal  studies. 
Further  details  are  given  in  the  section  dealing  with  the  skeleton. 


SENSE   ORGANS.  ?$ 

brane  is  placed  at  the  bottom  of  a  tube,  the  external  auditory 
meatus,  the  outer  end  of  which  is  frequently  protected  by 
movable  dermal  flaps.  In  most  mammals  an  external  ear,  sup- 
ported by  cartilages,  is  developed  ;  and  there  is  considerable 
evidence  to  show  that  this  external  ear  is  a  derivative  from  the 
operculum  of  fishes,  or  from  the  external  branchial  structures 
of  the  amphibia. 

Recent  experiments  tend  to  show  that  in  the  fishes  the  ears 
are  without  auditory  functions  and  are  solely  organs  of  equili- 
bration. In  terrestrial  vertebrates  they  are  both  organs  for 
hearing  and  for  the  maintenance  of  the  equilibrium. 

Olfactory  Organ.  —  The  organ  of  smell  is  a  single  sac  in  the 
cyclostomes,  paired  in  all  other  vertebrates.  Its  essential  por- 
tion is  the  sensory  epithelium,  in 
which  sensory  cells  are  inter- 
spersed with  supporting  or  isolat- 
ing cells.  Its  nerve  supply  is 
the  olfactory  nerve  already  de- 
scribed. The  powers  of  smell 
are  directly  proportional  to  the 
extent  of  sensory  surface,  and  in 
Order  that  this  may  be  increased 
the  surface  is  folded,  usually  in 
the  longitudinal  direction.  In  the 
more  primitive  forms  the  sensory 
surface  is  not  uniformly  distrib- 
uted, but  is  gathered  in  patches 
separated  by  large  masses  of  iso- 
lation cells.  In  some  ganoids 
and  amphibians  the  nasal  epithe-  uPPer  fisure  Acanthias>  showing  them 

..  ..  divided  by   a  movable   flap.     Lower, 

hum    has   a   peculiar  radiate  ap-    young  Amia.  ^  ^  anterior  and  pos. 
pearance,    as   seen   in   transverse    terior  narial  openings, 
section.      From  the  amphibia  up- 
wards outgrowths  of  cartilage  or  bone  (turbinals),  either  from 
the   ethmoid   or  lateral    walls,   tend   to   divide   the  cavity   still 
further.      In   the  petromyzontes  and   pisces  only  external  nos- 
trils occur,  and  in  the  cyclostomes  there  is  but  one  of  these.      In 
the  forms  with  paired  cavities  there  is  primitively  but  a  single 


FlG.  80.     Divided  nostrils  of  fishes ; 


76        MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


nostril  to  each  olfactory  sac,  but  in  the  selachians  and  ganoids 
a  fold  of  skin  practically  divides  each  nostril  (external  naris) 


FIG.  81.     Relations  of  nasal  organs  to  the  cavity  of  the  mouth;  A,  in  fishes; 
£,  in  terrestrial  vertebrates.     l>,  brain ;  i,  internal  nares  ;  w,  external  nares. 

into  two.  In  many  teleosts  this  is  carried  farther,  and  two  dis- 
tinct nostrils  may  occur  on  either  side.  These  modifications 
clearly  are  to  permit  a  current  of  water  over  the  olfactory 

epithelium  (Fig.  80). 

In  all  vertebrates  above  fishes 
both  external  and  internal  nares 
(choana)  are  present,  the  latter  open- 
ing into  the  oral  cavity.  This  condi- 
tion is  foreshadowed  in  the  selachians, 
where  an  oronasal  groove  leads  back 
from  the  external  nares  of  either  side 
to  the  angles  of  the  mouth.  In  the 
higher  vertebrates  this  groove  be- 
comes converted,  during  growth,  into 
a  tube  by  the  union  of  its  edges.1 
In  this  way  a  respiratory  tract  is 
formed  on  one  side  of  the  olfactory 

the  respiratory  nasal  tract  lead-       Surface,    the    posterior    end    of    which 

ing  from  the  nostril,  n,  into  the  opens  inside  the  cavity  of  the  mouth. 

In  a  similar  way  a  naso-lachrymal 
duct  is  formed  leading  from  each  eye 
into  the  corresponding  nasal  passage. 
In  terrestrial  vertebrates  nasal  glands 
are  frequently  present  in  connection 

with  the  nose,  the   secretion   of  which  moistens  the  olfactory 

epithelium. 

1  The  process  is  modified  in  certain  groups,  where  a  solid  cord  of  cells,  instead  of  a 
groove,  is  formed,  the  respiratory  passage  appearing  later  in  the  cord. 


FIG.  82.  Head  region  of  a 
human  embryo,  after  His,  show- 
ing the  method  of  formation  of 


oral  cavity,  and  the  naso-lach- 
rymal duct  leading  from  the  eye, 
e,  into  the  nasal  cavity.  />, 
rudimentary  gill  clefts  ;  //,  hypo- 
physial  pocket ;  /,  lungs ;  s,  cer- 
vical sinus. 


SEA'SE   ORGANS.  77 

Connected  with  the  nose  in  all  vertebrates  above  the  fishes 
is  a  pair  of  accessory  sensory  organs,  —  the  organs  of  Jacobson. 
They  are  outpushings  of  the  wall  of  the  olfactory  surface,  sup- 
plied by  branches  of  the  first  and  fifth  nerves.  In  the  lower 
amphibia  these  organs  are  placed  on  the  medial  side  of  the  nasal 
cavities  ;  a  little  higher  they  are  ventral  in  position  ;  in  the  high- 
est amphibia  they  have  rotated  to  the  lateral  side  of  the  olfactory 
organ.  In  the  amniotes  they  are  either  medial  or  ventral  in 
position.  In  the  lower  forms  these  sacs  are  connected  only 


FIG.  83.  Section  through  the  nasal  region  of  the  Surinam  toad,  Pipa.  t,  car- 
tilage; en,  cavum  nasale ;  d,  Jacobson's  gland  and  duct;  e,  ethmoid  cartilage; 
/,  frontal  bone ;  /,  organ  of  Jacobson ;  /,  lateral  portion  of  nasal  passage ;  «,  nasal 
bone ;  «/,  branch  of  olfactory  nerve  to  organ  of  Jacobson ;  nn,  branches  of  nasal 
nerve  of  trigeminal. 

with  the  nasal  cavities  ;  but  in  the  mammals  a  duct  (Stenson's 
duct)  sometimes  leads  from  them  into  the  mouth  through  the 
foramina  incisiva,  between  the  premaxillary  and  the  palatine 
processes  of  the  maxillary  bones.  In  many  mammals,  however, 
these  foramina  are  closed  by  membrane,  and  are  vestigial  in 
character. 

In  the  mammals  for  the  first  time  appears  an  external  nose 
supported  by  cartilage.  In  some,  like  the  tapirs  and  elephants, 
this  organ  becomes  enormously  developed,  and  forms  in  the 
latter  the  well-known  trunk. 


78        MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


Visual  Organs.  —  The  sensory  portion  of  the  eyes  arises  from 
the  brain,  and  in  the  embryos  of  some  vertebrates  (elasmo- 
branchs,  urodeles)  optic  areas  can  be  recognized  in  the  medullary 


FIG.  84.  Diagrams  showing  the  inversion  of  the  layers  in  the  formation  of  the 
retina.  In  all  the  figures  the  nuclei  are  placed  in  the  morphologically  deeper  ends 
of  the  cells.  In  A  the  brain  (£)  has  been  closed  in,  in  B  the  optic  vesicle  (z>)  has 
reached  the  thickening  for  the  lens  (/),  and  on  the  right  side  the  vesicle  is  being  con- 
verted into  the  double-walled  cup  with,  as  shown  in  C,  a  medial  epithelial  (<?),  and 
an  outer  retinal  layer  (r),  the  deeper  face  of  which  is  turned  towards  the  lens. 

plate  before  its  involution.     The  accessory  portions  are  furnished 

by  ectoderm,  mesothelium,  and  mesenchyme. 

A  hollow  outgrowth  arises  on  either  side  of  the  primitive 

fore  brain,  and  extends  towards  the  skin.     The  distal  portion 

expands  into  a  globular  optic 
vesicle,  while  the  proximal  por- 
tion retains  its  smaller  size,  and 
is  known  as  the  optic  stalk. 
Thus  the  cavity  of  the  vesicle 
is  in  connection  with  the  ven- 
tricle of  the  thalamencephalon 
by  means  of  the  hollow  stalk. 
The  distal  surface  of  each 
optic  vesicle  comes  in  contact 
with  the  ectoderm  of  the  side 
of  the  head  at  the  place  where 
the  lens  is  to  form  (see  below), 
and  with  the  formation  of  this 


FIG.  85.  Diagram  of  early  develop- 
ment of  eye,  modified  from  Hertwig.  b, 
blood-vessels  ;  c,  cavity  of  optic  vesicle ; 
^,  epithelial  layer  ;/,  choroid  fissure;  /, 
lens;  r,  retinal  layer;  v,  cavity  of  optic 
cup,  later  occupied  by  vitreous  body. 


structure  the  distal  half  of  the 

optic  vesicle  becomes  invaginated  into  the  proximal  part,  thus 
partially  obliterating  the  cavity  of  the  vesicle,  and  converting  the 
whole  into  a  two-layered  cup.  The  distal  invaginated  part  of 


SENSE   ORGANS.  79 

this  cup  eventually  becomes  the  retina,  while  the  outer  layer 
forms  the  pigmented  epithelium  (pigment  layer  of  the  choroid 
of  older  works)  of  the  eye. 

Connected  with  the  invagination  of  the  retinal  layer  is 
another  phenomenon,  an  account  of  which  is  necessary  for  the 
understanding  of  other  features  of  the  eye.  This  invagination 
is  not  confined  to  the  distal  portion  of  the  optic  vesicle,  but  ex- 
tends along  its  lower  surface  and  continues  upon  the  optic  stalk 
in  a  manner  readily  understood  from  Fig.  85,  the  result  being 
a  gap,  the  choroid  fissure,  in  the  ventral  wall  of  the  optic  cup, 
produced  as  a  groove  along  the  lower  side  of  the  optic  stalk. 

Through  this  choroid  fissure  mesenchyme  cells,  and  later 
blood-vessels,  enter  the  optic  cup.  Later,  when  the  fissure 
closes,  the  walls  of  the  stalk  unite  around  the  blood-vessels, 
which  hence,  apparently,  enter  the  optic  cup  through  the  centre 
of  the  optic  stalk.  A  loose  watery  tissue,  the  vitreous  humor, 
is  developed  from  the  immigrant  mesenchyme  cells,  and  fills  the 
optic  cup,  or  as  it  is  called  in  the  adult,  the  posterior  chamber,  of 
the  eye.  The  blood-vessels  serve  to  nourish  the  retina,  etc. 

At  first  the  retinal  layer  is  thin  ;  but  it  gradually  increases  in 
thickness  by  cell  division  so  that  it  eventually  consists  of  several 
layers  of  cells,  and  finally  these  become  differentiated  so  that 
several  strata  can  be  distinguished.  Those  nearest  the  lens 
become  the  ganglion  cells,  those  farthest  away  the  rod-  and  cone- 
cells,  and  between  these  a  so-called  granular  layer,  this  last 
being  separated  from  the  other  two  by  an  inner  and  outer  mo- 
lecular layer.  From  some  of  the  rod-  and  cone-cells,  which  are 
the  sensory  strictures  of  the  eyes,  slender  rods  and  cones  (Fig. 
66),  grow  out  towards  and  into  the  pigmented  epithelium,  while 
others  of  this  layer  develop  into  supporting  or  isolating  cells. 
From  the  other  side  of  each  rod  cell  and  cone  cell  a  nerve  fibre 
grows  out  towards  the  front  of  the  eye,  and  breaks  up  into 
dendrites  which  interlace  with  other  dendrites  coming  from  the 
cells  of  the  granular  layer,  their  fibrillations  producing  the  inner 
molecular  layer.  The  outer  molecular  layer  is  similarly  an  in- 
terlacing of  dendrites  from  the  granular  cells  and  from  the  gan- 
glion cells,  the  minute  granulations  which  occasioned  the  name 
molecular  layer  being  the  sections  of  the  nerve  fibrillations.  The 


8o 


MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


ganglion  cells  in  their  turn  produce  from  these  outer  or  free 
surfaces  axons  which  rapidly  grow  from  the  cells  to  the  choroid 
fissure  (thus  forming  a  layer  of  nerve  fibres  over  the  ganglion 
cells),  and  thence,  through  the  groove  in  the  optic  stalk,  to  the 
brain.  These  axons  form  the  optic  nerve,1  which,  as  will  readily 


FIG.  86.  Retinal  elements,  after  Ramon  yCajal.  BC,  bipolar  cone  cell;  BR, 
bipolar  rod  cell;  C,  cone;  CF,  centrifugal  fibre;  G,  ganglion  cell  layer;  H,  hori- 
zontal cell;  76",  inner  granular  layer  ;  IM,  inner  molecular  layer;  N,  nerve  fibres  ; 
OG,  outer  granular  layer  ;  OAf,  outer  molecular  layer;  /v',  rods;  S,  supporting  cells. 

be  understood,  appears,  after  the  closure  of  the  choroid  fissure, 
as  if  it  left  the  eye  through  the  centre  of  the  retina.  As  all 
sense  cells  are  lacking  at  the  point  of  exit  of  the  optic  nerve, 
this  region  forms  the  l  blind  spot '  described  in  all  physiological 
text-books. 

1  It  was  formerly  thought  that  the  optic  nerve  arose  by  a  modification  of  the  cells  of 
the  optic  stalk.  Later,  nerve  fibres  were  described  as  growing  from  the  brain  to  the  eye  ;  but 
while  some  fibres  may  arise  in  this  way,  the  majority  arise  as  described  above.  So  far  as 
method  of  nerve  formation  is  concerned,  the  optic  nerve  resembles  the  dorsal  root  of  a  spinal 
nerve  (pp.  47  and  60,  foot-note.) 


SENSE   ORGANS.  8 1 

One  point  regarding  the  eye  was  formerly  emphasized.  As 
will  be  seen  from  the  method  of  formation  of  the  eye  by  invo- 
lution from  the  skin  (see  Fig.  84),  the  layer  of  rods  and  cones 
is  homologous  with  the  superficial  layer  of  the  skin,  while  the 
ganglionic  layer  corresponds  to  the  deeper  surface  of  the  epi- 
dermis. Hence  light  passing  into  the  eye  transverses  the 
transparent  deeper  layers  in  order  to  reach  the  morphologically 
superficial  sense  structures,  the  rods  and  cones,1  a  condition 
which  is  unlike  that  occurring  in  any  invertebrate  eye,  with  the 
exception  of  the  peculiar  dorsal  eyes  described  as  occurring  in 
the  slug  Onchidium. 

At  the  place  where  the  optic  vesicle  reaches  the  ectoderm 
of  the  side  of  the  head,  the  latter  thickens,  and  then  a  portion 
of  it  becomes  invaginated,  and  is 
at  last  cut  off  as  an  epithelial  sac, 
-  the  vesicle  of  the  lens.  This 
body,  which  comes  to  lie  in  the 
aperture  of  the  optic  cup,  has  an 
anterior  wall  of  cubical  cells,  while 
those  of  the  posterior  surface  are 
so  strongly  columnar  that  the  cav- 
ity is  nearly  obliterated.  With 
growth  the  cavity  entirely  disap- 
pears, while  the  lens  of  the  adult 

is  developed  by  the  addition  of  FlG.  g7<  Development  of  eye  in 
elongate  fibres  produced  by  bud-  pig.  f,  pigmented  epithelium;  /, 

ding  from  the  cells  of  the  equator   lens'  m>  ™esenchy™e;  «,  nervous 

_  layer  of  retina  ;   r,   deeper  layer  of 

of  the  structure.     These  fibres  are   retina;  .r,  optic  stalk. 
arranged  in  layers,  like  the  coats 

of  an  onion,  and  where  they  meet  on  the  inner  and  outer  sur- 
faces of  the  lens,  they  produce  peculiar  figures  like  a  three-rayed 
star. 

After  the  lens  is  cut  off  from  the  ectoderm  the  latter  be- 
comes a  smooth,  transparent  sheet  over  the  front  of  the  eye, 
forming  the  epithelium  known  as  the  conjunctiva,  continuous 
with  the  superficial  layer  of  the  skin. 

3  In  man  there  are  from  250,000  to  1,000,000  rods  and  cones  to  a  square  millimetre  of 
retinal  surface. 


82        MORPHOLOGY  OF   THE    ORGANS  OF  VERTEBRATES. 

We  are  now  in  position  to  describe  the  eye  of  the  adult  ver- 
tebrate. The  eye  proper  is  approximately  spherical,  although, 
as  in  fishes,  it  may  be  flattened,  or,  as  in  birds,  somewhat  coni- 
cal in  front.  In  the  ichthyopsida  it  is  without  any  well-devel- 
oped external  accessories  for  protection  ; l  but  in  the  amniotes 


n  sh 


FIG.  88.  Horizontal  section  through  human  eye,  from  Hertwig  after  Arlt.  a, 
arteria  centralis  ;  ac,  anterior  chamber  of  eye;  r,  cornea;  ch,  choroid;  cj,  conjunc- 
tiva; cp,  ciliary  process;  z,  iris;  /,  lens;  m,  macula  lutea,  point  of  distinct  vision; 
n,  optic  nerve  ;  a,  ora  serrata ;  /,  papilla  of  optic  nerve ;  pc^  posterior  chamber  of 
eye;  r,  retina;  5,  sclerotic;  sht  sheath  of  optic  nerve;  v,  vitreous  body ;  z,  zonula 
Zinnii. 

movable  lids,  which  can  close  over  the  organ,  occur.  There 
are  typically  three  of  these  folds  of  the  skin,  —  an  upper  and  a 
lower  lid,  moving  in  a  vertical  plane,  and  inside  of  these  a  third 
transparent  lid,  the  nictitating  membrane,  which  is  attached  at 
the  anterior  or  inner  angle,  and  which  closes  horizontally.  In 

1  Some  salamanders  have  feebly  developed  eyelids. 


SENSE   ORGANS.  83 

man  the  nictitating  membrane  is  reduced  to  a  vestigial  fold,  the 
plica  semilunaris,  visible  at  the  inner  angle  of  the  eye. 

The  free  surface  of  the  eye  is  covered  by  the  conjunctiva 
already  mentioned ;  and  beneath  this  is  a  thicker,  dense  trans- 
parent layer,  the  cornea,  composed  of  connective  tissue  fibres 
produced  from  mesenchyme  cells,  which  penetrate  between  the 
conjunctiva  and  the  lens.  Laterally  the  cornea  is  continuous 
with  a  hard  white  capsule,  the  sclerotic  coat,  which  envelops 
the  whole  eyeball,  its  anterior  portion  being  the  well-known 
'  white  of  the  eye.'  This  sclerotic  is  usually  cartilaginous,  and 
in  the  sauropsida  and  in  monotremes,  bony  structures,  sclerotic 
bones,  may  be  developed  in  it.  This  sclerotic  forms  a  sense 
capsule,  comparable  in  a  way  to  those  enclosing  the  ears 
and  olfactory  organs,  but  never,  like  them,  uniting  with  the 
skull. 

Between  the  cornea  and  the  lens  is  the  anterior  chamber  of 
the  eye,  filled  with  a  watery  fluid,  the  aqueous  humor,  less  dense 
than  the  vitreous  humor  already  mentioned. 

Inside  of  the  sclerotic  is  a  highly  vascular  layer,  the  choroid, 
which  carries  numerous  blood-vessels  to  nourish  the  eye.  The 
choroid  extends  forward  nearly  to  the  edge  of  the  optic  cup  ;  but 
beyond  this  point  it  becomes  muscular,  a  portion  of  it  forming 
the  contractile  portion  of  a  circular  curtain,  which  extends  from 
the  edge  of  the  optic  cup  into  the  anterior  chamber.  This 
curtain,  known  as  the  iris,  is  opaque,  and  is  usually  colored  by 
pigment  derived  from  the  edge  of  the  optic  cup.  The  opening 
in  the  centre  of  the  iris,  the  pupil,  can  be  enlarged  or  contracted 
by  means  of  the  muscles  already  referred  to,  and  thus  the  amount 
of  light  admitted  to  the  retina  can  be  regulated. 

Just  inside  the  iris  the  inner  wall  of  the  optic  cup  becomes 
developed  into  a  strong  ridge,  the  ciliary  process,  which  ex- 
tends inwards  towards  the  lens  to  which  it  is  attached  by  a 
feriestrated,  suspensory  ligament  (zonula  Zinnii),  thus  partially 
separating  the  anterior  from  the  posterior  chambers.  Close  to 
this  region  the  choroid  develops  a  layer  of  ciliary  muscles, 
which  by  their  action  can  move  the  lens  nearer  to  or  farther 
from  the  retina,  and  at  the  same  time,  by  stress  conveyed  by 
the  suspensory  ligament,  can  slightly  alter  its  shape.  This 


84        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


forms  the  apparatus  of  accommodation  necessary  for  viewing 
objects  at  different  distances. 

In  the  lower  vertebrates  accessory  glands  connected  with 
the  eyes  are  but  slightly  developed,  but  with  the  assumption  of 
a  terrestrial  life  (amphibia)  lachrymal  glands  for  lubricating 
the  surface  appear.  These  arise  as  inpushings  of  the  epider- 
mis near  the  lids.  In  the  lower  amphibia  the  glands  are  on  the 
lower  side  of  the  eye  and  form  a  continuous  series ;  but  higher 
this  becomes  divided  into  two,  —  a  Haider's  gland  near  the 
inner  angle,  a  true  lachrymal  gland  at  the  outer.  In  reptiles l 
and  birds  these  remain  on  the  lower  side  of  the  eye,  but  in 
mammals  the  lachrymal  gland  passes  to  the  upper  lid.  The 
Harderian  gland,  which  has  for  its  Junction  the  lubrication  of 
the  nictitating  membrane,  becomes  reduced  in  the  mammalia. 
The  lachrymal  duct  has  already  been  mentioned  (p.  76). 

The  eye  is  provided  with  muscles  which  move  it  as  a  whole. 
Some  of  these  are  remarkably  constant  through  the  whole  verte- 
brate series.  There  are  four  rectus 
muscles,  known  from  their  position  as 
the  superior,  inferior,  external,  and  in- 
ternal. These  arise  from  the  bottom 
of  the  orbit  near  the  foramen  for  the 
optic  nerve,  and  are  inserted  at  about 
equal  distances  around  the  ball.  The 
two  oblique  muscles  (superior  and  in- 
ferior) arise  in  front  of  the  rectus  mus- 
cles, and  are  inserted  on  the  ball  above 
and  below  the  internal  rectus.  Besides, 
there  may  be  a  well-developed  retractor 
bulbi  attached  near  the  optic  nerve, 
and  serving  to  pull  the  eye  back  into 
its  socket.  In  the  sauropsida  are  also 
muscles  connected  with  the  nictitating 
membrane,  but  these  are  reduced  or  ab- 
sorbed in  the  mammals. 

Epiphysial  Structures.  —  Several  structures  which  are  con- 
nected with  that  part  of  the  primitive  fore  brain  which  subse- 

i  An  interesting  fact  is  the  absence  of  lachrymal  glands  in  crocodiles. 


FIG.  89.  Eye  muscles 
and  related  nerves  in  shark. 
a,  abducens  nerve ;  oi,  infe- 
rior oblique ;  os,  superior 
oblique;  om,  oculomotor 
nerve  ;  re,  rectus  externus ; 
ri,  r.  internus;  rif,  r.  infe- 
rior ;  rs,  r.  superior  muscles. 


SENSE    ORGANS.  85 

quently  becomes  the  twixt  brain  are  best  considered  in 
connection  with  the  sense  organs.  These  are  best  developed 
in  the  lizards,  and  hence  these  animals  serve  as  the  basis  of  the 
following  account.  At  an  early  stage  there  arises  from  the 
epithelial  roof  of  this  region  a  hollow  outgrowth  (the  epiphysis) 
directed  upwards  and  forwards,  its  distal  end  at  first  being  in 
contact  with  the  epidermis  of  the  top  of  the  head.  The  extrem- 


yjC^'lT' 
i^titfH 


-vr._.  — a 


FIG.  90.     Section  of  pineal  eye  of  Ilatteria^  after  Spencer  from  Wiedersheim. 
a,   capsule  ;  b,  lens  ;    <:,  vesicle  ;    </,  retina  ;    ey  molecular  layer  ;  ft  blood-vessels  ; 
g,  large  cells  ;   A,  nerve. 

ity  of  this  outgrowth  expands  into  a  more  or  less  spherical 
vesicle,  the  pineal  or  parietal  eye,  which  may  come  to  lie  at  some 
distance  in  front  of  its  point  of  origin,  above  the  cerebral  hemi- 
sphere, and  usually  on  the  right  side.  The  cells  of  this  pineal 
vesicle  increase  in  number,  producing  a  thickening  of  the  walls  ; 
while  the  distal  and  proximal  surfaces  of  the  vesicle  become 


86       MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


differentiated,  the  former  into  a  transparent  lens-like  body,  the 
latter  into  sense  cells,  supporting  cells,  and  pigment  cells,  the 
whole  making  up  a  retina.  Thus  is  formed  a  camera  eye  ;  and 
in  the  lizards  this  comes  to  lie  on  the  top 
of  the  head  just  beneath  the  skin,  one  of 
the  plates  of  the  dorsal  surface  bearing  a 
transparent  spot  through  which  light  can 
reach  the  organ.  This  parietal  eye,  how- 
ever, differs  from  the  paired  eyes  already 
described  in  the  relations  of  the  nerves  to 
the  retina  (p.  81).  These  proceed  from  the 
deeper  ends  of  the  sense  cells,  but  it  must 
be  kept  in  mind  that  there  has  been  no  inver- 
sion of  the  retinal  layer  in  the  parietal  eye. 
After  the  parietal  eye  has  been  budded 
off  from  the  epiphysis,  there  is  frequently 
formed  a  less  perfect  eye-like  organ,  the 

parapinealis,  from  the  distal  end  of  the  stalk.     In  some  cases 
the    epiphysis    is    double,    in    which    case    the   pinealis    arises 


FIG.  91.  Dorsal  view 
of  head  of  Sceleporus  un- 
dulatus;  p, parietal  organ. 


7 


FlG.  92.     Pineal  apparatus  in  an  embryo  lizard  {Sceleporus}.     b,  blood-vessels; 
c,  cerebrum;  e,  epiphysis;  />,  parietal  eye;  pa,  paraphysis;   //,  parapinealis. 


EPIDERMAL   STRUCTURES,  87 

from  the  anterior,  the  parapinealis  from  the  posterior,  out- 
growth. Besides  these,  a  third  outgrowth,  the  paraphysis,  may 
arise  in  front  of  the  epiphysis ;  it  never  develops  sensory 
elements. 

The  parietal  eye  has  different  fates  even  in  the  lizards.  In 
some,  as  Hatteria  (Fig.  90),  it  retains  its  parts  well  developed 
throughout  life,  and  its  nervous  connection  with  the  brain  per- 
sists, so  that  here  it  is  apparently  to  some  extent  functional. 
In  other  lizards  it  has  lost  this  power,  either  through  the  exten- 
sive deposit  of  pigment  in  all  parts,  or  by  the  degeneration  of 
its  nerve  supply.  In  other  groups  these  structures  of  the  pin- 
eal apparatus  are  much  more  rudimentary,  and  are  usually 
entirely  enclosed  within  the  skull ;  although  at  times,  as  in  the 
anura,  the  parietal  eye  may  lie  between  the  bones  and  the  skin, 
but  here  all  nervous  connection  is  lost.  In  many  extinct 
vertebrates,  if  we  may  judge  by  the  large  parietal  foramen  in 
the  skull,  the  pinealis  was  well  developed  and  functional. 


EPIDERMAL    AND    DERMAL    STRUCTURES    (SKIN). 

The  Skin.  —  That  portion  of  the  ectoderm  which  remains 
on  the  surface  of  the  embryo  after  the  differentiation  and  in- 
volution of  the  central  nervous  system  forms  the  epidermis, 
which,  together  with  the  underlying  mesenchymatous  tissue,  the 
derma,  makes  up  the  skin.  In  its  earliest  stage  the  epidermis 
is  usually  but  a  single  cell  in  thickness,  but  later,  by  division  of 
these  cells,  other  layers  are  formed  on  the  outside  of  this  first  or 
basal  layer.  In  ganoids,  teleosts,  and  amphibia  the  epidermis 
is  two  cells  thick  from  the  first,  and  in  the  amphibia  (the  only 
instance  in  vertebrates)  the  outer  layer  is  ciliated  in  the  young. 
The  basal  layer  is  the  active  portion,  producing  by  cell  division 
the  more  superficial  layers.  In  those  forms  with  two-layered 
epidermis,  the  basal  layer  also  gives  rise  to  nerves  and  sense 
organs,  and  is  therefore  often  spoken  of  as  the  nervous  layer, 
the  outer  one  being  the  cuticular  layer. 

The  derma  (often  called  cutis  or  corium)  is  of  mesenchy- 
matous origin,  and  consists  largely  of  layers  of  fibrous  connec- 


88 


MORPHOLOGY  OF  THE  ORGANS  OF  VERTEBRATES. 


tive  tissue  intermingled  with  smooth  muscle  cells,  blood-vessels, 
nerves,  etc.,  the  whole  being  separated  from  the  deeper  tissues 
by  a  layer  of  much  looser  connective  tissue,  usually  containing 
considerable  fat. 

The  structure  of  the  adult  epidermis  varies  considerably  in 
the  different  groups.     In  all  it  becomes  several  layers  in  thick- 
ness,  and  is  thicker   in 

*      3        $  cf  the  terrestrial  than  in  the 

aquatic   forms.      In   the 
ichthyopsida     there    is 
slight  differentiation  be- 
tween   the    layers,    the 
cells  showing  less  strat- 
ification    than     in     the 
higher  groups.     Among 
them  are  numerous  uni- 
cellular   glands,   usually 
spherical   in   shape,  and 
loaded  with  a  slimy  sub- 
stance (mucus)  ;  and  as 
these  cells  approach  the 
surface  they  break,   and 
their     contents     spread 
over  the  body,  producing 
the    slimy   condition   so 
familiar  in  these  forms. 
In  the  amniotes,  on  the 
other    hand,    the    outer 
layers  of  the  epidermis 
undergo  a  hardening  pro- 
cess, and  are  converted 
into  a  horny  layer  (stra- 
tum corneum),  Fig.  95,  the  beginnings  of  which  are  seen  in  the 
frogs.     Apparently  the  first  layer  to  be  budded  from  the  basal 
layer  persists  through  a  large  part  of  the  embryonic  life  as  a 
distinct   sheet  on  the  outside  of  the  corneum,  known   as  the 
epitrichium,  so  called  because  in  embryonic  mammals  it  extends 
in  an  unbroken  sheet  over  the  developing  hairs.     The  non-corni- 


Co 


FIG.  93.  Section  of  skin  of  lamprey  eel 
(  Petromyzon  planert)  from  Wiedersheim.  B, 
mucous  cells ;  Co,  derma ;  CS,  cuticular  layer  ; 
Ep,  epidermis ;  F,  fat ;  G,  blood-vessels ;  Ko, 
club  cells  ;  Kb,  granular  cells  ;  S,  IV,  fibres  of  con- 
nective tissue  running  vertically  and  horizontally. 


EPIDERMAL   STRUCTURES. 


89 


fied  layers  of  cells  are  known  as  the  Malpighian  layer,  the  cells 
of  which  are  polygonal  in  outline  and  rich  in  protoplasm      In 
the  mammals,  be- 
tween horny  and 
Malpighian      lay- 
ers, is  a  thin  stra- 
tum   lucidum, 
consisting  of  ex- 


FiG.  94.  Skin  of  mammalian  embryo,  showing  the 
epitrichium,  e,  after  Minot.  b,  basal  layer  ;  m,  Malpighian 
layer. 


tremely  flattened 

cells  closely  compacted  together. 

As  will  readily  be  understood,  the  cells  of  the  basal  layer 

are  continually  dividing,  thus  producing  new  cells,  which  come 

to  lie  between  the  basal 
layer  and  those  layers 
previously  formed,  and 
in  this  way  tending  to 
increase  the  thickness 
of  the  epidermis.  As 
these  cells  grow  older 
they  gradually  pass  into 
the  conditions  found  in 
the  different  layers,  — 
lucidum,  corneum,  — 
and  at  last  are  cast  off 
from  the  outer  surface, 
either  a  few  cells  at  a 
time,  or  in  larger  sheets, 

FIG.  95.     Diagrammatic  section  through  mam-  as     in     the     amphibians 

malian  skin.      C,  stratum  corneum;    Z>,  derma;  and  reptiles.    The  Strata 

G,  sweat  gland;    //,  hair;    L,  stratum  lucidum;  corneum     and     lucidum 
A/,    Malpighian    layer  ;    A7,    nerve ;     T,    tactile 

corpuscles.  are    clearly    protective 

in  nature,  and  only  in 

the  turtles  is  there  an  absence  of  this  shedding  of  the  external 
layers  of  the  skin. 

Dermal  Glands.  —  The  epidermis  gives  rise  to  numerous 
glands.  In  the  fishes  the  unicellular  mucous  glands  have  already 
been  referred  to.  In  some  fishes  (teleosts)  in  addition  the 
skin  also  contains  poison  glands,  sometimes  upon  the  back, 


90        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


sometimes  on  the  operculum,  and  sometimes  in  the  axilla,  but 
always  in  connection  with  a  strong  spine.  Of  our  native  fishes 
the  poison  glands  in  the  axilla  of  certain  catfishes  {Notunts)  are 
best  known.  In  the  toadfish  (^Batrachus)  a  gland  in  similar 
position  is  well  known,  but  apparently  its  secretion  is  not 
poisonous. 

In  the  amphibia,  glands  in  the  skin  which  secrete  an  acrid 
juice  are  abundant,  and  in  the  toads  their  presence  causes  the 
warty  skin  so  noticeable  in  these  animals.  In  the  sauropsida, 
glands  are  few  in  number.  In  certain  snakes  stink  glands  occur 
in  the  skin,  the  secretions  of  which  give  these  animals  a  dis- 
agreeable odor.  In  the  lizards,  glands  are  found  only  on  the 
inside  of  the  femoral  region  of  the  hind  limbs,  the  openings  of 
which  (femoral  pores)  are  of  considerable  value  in  the  classifi- 
cation of  these  forms.  In  the  birds,  glands  are  developed  on 
the  reduced  tail  (uropygial  glands),  the  oily  secretion  of  which 
is  used  in  oiling  the  feathers.  These  glands  are  best  developed 
in  the  water  birds.  In  the  rasores  there  are  in  addition  glands 
in  the  neighborhood  of  the  eye. 

In  the  mammals,  glands  are  well  developed,  and  acquire  a 
great  variety  of  form.  These  glands  may  be  arranged  in  two 

categories,  the  tubular 
and  the  racemose,  the 
characters  of  which 
are  indicated  by  their 
names.  To  the  tubu- 
lar type  belong  the 
sweat  glands,  which 
extend  deep  into  the 
derma,  and  in  their 
deeper  portions  be- 
come coiled  and  con- 
voluted (Fig.  95). 
The  racemose  (acinose)  glands,  in  their  simplest  condition, 
form  the  sebaceous  glands,  and  are  normally  placed  in  close 
-connection  with  the  roots  of  the  hair.  In  some  mammals  the 
sebaceous  glands  of  certain  regions  of  the  body  become  con- 
verted into  scent  glands,  the  secretions  of  which  may  serve  for 


TIG.  96.  Different  types  of  glands,  a,  intesti- 
nal epkhelium  with  a  gland- (goblet)  cell;  b,  uni- 
cellular gland  with  duct ;  c,  simple  tubular  gland  ; 
J,  simple  racemose  gland ;  e,  compound  racemose 
gland. 


EPIDERMAL   STRUCTURES. 


offence  or  defence ;  or  again  may  be  of  value  at  the  rutting 
season,  as  attractions  for  the  other  sex.  Of  the  defensive 
glands  those  of  the  skunk  and  polecat  come  immediately  to 
mind ;  to  the  other  category  belong  the  peculiar  glands  of  the 
beaver,  civet  cat,  musk  deer,  etc.  These  glands  may  be  placed 
near  the  eyes  (deer),  on  the  back  (musk  swine),  on  the  legs 
(ordinary  swine),  on  the  ventral  surface  (musk  deer),  or  near 
the  vent  (skunks,  etc.). 

A  more  extreme  modification  of  'the  sel^aceous  glands  is 
found  in  the  milk  glands  of  all  mammals  except  monotremes,1 
the  secretion  of  which 
serves  to  nourish  the 
young.  These  milk 
glands  are  in  pairs 
upon  the  ventral  sur- 
face, the  number  being 
roughly  correlated  to 
the  number  of  young 
brought  forth  at  a  time. 
The  ducts  of  each 
group  of  glands  open 
upon  a  limited  extent  of  surface,  and  this  becomes  converted 
into  a  teat  or  nipple,  either  by  the  elevation  of  the  skin  in 
which  the  ducts  open  (Fig.  97,  A},  or  by  the  extension  of  the 
surrounding  skin  into  a  tubular  form  (B). 

In  the  skin,  pigment  is  of  common  occurrence.  It  may  be 
found  in  the  epidermis,  but  is  more  common  in  the  derma.  It 
may  consist  of  scattered  pigment  granules,  or  there  may  be 
special  pigment  cells  (chromatophores)  which  enlarge  or  con- 
tract under  control  of  the  nervous  system,  producing  those 
color  changes  so  noticeable  in  many  lizards,  and  to  a  less  degree 
in  amphibia  and  fishes. 

EXOSKELETON. 

Either  or  both  layers  of  the  skin  may  participate  in  the 
formation  of  firmer  parts  constituting  a  tegumentary  skeleton, 
which  may  take  the  form  of  scales  or  bony  plates  ;  and  it  is  to 

1  The  milk  glands  of  the  monotremes  are  apparently  derivatives  of  sweat  glands. 


FIG.  97.     Two  types  of  nipple. 


92        MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

be  noted  that  most  of  the  membrane  bones  of  the  skull  (infra) 
belong  in  this  category.  Here,  too,  may  be  enumerated  feathers, 
hair,  horn,  claws,  etc.,  as  well  as  cornifications  of  the  skin  of 
more  limited  distribution. 

Scales.  —  The  most  primitive  type  of  this  exoskeleton  is 
found  in  the  scales  of  the  elasmobranchs.  Here  papillae  of  the 
derma  (dentinal  papillae),  arranged  in  quincunx,  push  up  into 
the  epidermis,  carrying  the  basal  layer  of  the  latter  before  them. 
The  external  surface  of  each  papilla  and  its  base  secretes  a  little 
plate  of  bone  or  dentine  with  a  central  spine ;  while  the  epi- 
dermis covering  the  papilla  becomes  converted  into  an  enamel 


FIG.  98.  Developing  scales  of  dogfish  (Aca.nthi.as~}.  &,  basal  layer  of  ecto- 
derm; c.  derma  (corium) ;  </,  dentine;  e,  enamel;  eo,  enamel  organ;  /,  pulp. 

organ,  the  deeper  face  of  which  secretes  a  hard  enamel  cap  upon 
the  dentine  base,  the  enamel  being  thickest  upon  the  central 
spine.  These  scales  are  known  as  placoid  scales,  and  in  their 
development  they  show  the  closest  similarity  with  teeth  (see 
p.  19). 

In  the  ganoids  (Lepidosteus)  the  early  development  is  as  in 
elasmobranchs,  including  the  formation  of  plate,  spines,  and  a 
rudimentary  enamel  cap.  Later  the  spines  and  enamel  cap  dis- 
appear, while  the  outer  side  of  the  dentinal  plate  becomes  cov- 
ered by  a  hard,  smooth  layer  known  as  ganoin,  which  differs 
from  enamel  in  that  it  arises  from  the  derma.  In  the  higher 
ganoids  and  in  the  teleosts,  dentinal  papillae  are  formed  ;  but  the 
resulting  scales  are  entirely  of  dermal  origin,  and,  whether  soft 
and  flexible,  hard  and  bony,  show  no  differentiation  into  layers. 
At  first  these  scales  are  arranged  in  quincunx  parallel  to  thje 


EPIDERMAL  STRUCTURES.  93 

external  surface  of  the  body,  but  usually  with  growth  the  ante- 
rior end  of  each  extends  beneath  the  posterior  margin  of  the 
scale  in  front,  so  that  as  a  result  the  scales  come  to  lie  in  dermal 
pockets  oblique  to  the  surface.  In  some  cases  (South  American 
siluroids,  many"  plectognaths,  etc.)  the  scales  may  fuse  into  a 
firm  dermal  armor  enclosing  the  body,  while  in  many  fossil  gan- 
oids this  external 
skeletonwas 
highly  developed; 
the  parts  uniting 

in  SOme  instances  FIG.  99.     Position  of  scales  (black)  in  skin  of 

into    large    armor  teleost.     A  derma;  Ey  epidermis. 

plates. 

All  existing  amphibia  except  some  caecilians  are  without 
scales.1  In  the  latter  group  and  in  many  fossil  amphibia  they  are 
(or  were)  well  developed.  In  caecilians  the  scales  are  dermal, 
and  lie  in  the  rings  which  encircle  the  body.  In  the  stegoceph- 
alans  these  plates  were  in  some  cases  confined  to  the  ventral 
surface,  in  some  they  covered  the  entire  body. 

In  the  reptilia  of  all  groups  forms  are  found  with  a  well- 
developed  dermal  skeleton  of  bony  plates,  the  plates  in  Stegosan- 
rus  (one  of  the  extinct  dinosaurs)  being  nearly  two  feet  across. 
In  recent  forms  similar  but  smaller  dermal  bones  occur  in  alli- 
gators and  many  lizards,  and  reach  their  extreme  in  the  turtles, 
where  these  bony  plates  unite  to  form  a  bony  box,  composed  of 
an  upper  carapace  and  a  lower  plastron,  enclosing  the  trunk. 
This  shell  becomes  firmly  united  with  the  true  skeleton,  and  to 
a  certain  extent  replaces  it  in  some  species,  the  vertebras  and 
parts  of  the  ribs  being  correspondingly  reduced. 

Besides  this  bony  skeleton  reptiles  are  also  provided  with 
scales,  in  the  formation  of  which  a  papilla  of  the  derma  is  formed, 
but  the  scales  themselves  arise  from  cornifications  of  the  outer 
epidermal  cells.  This  horny  envelope  is  periodical!^  moulted  by 
all  reptiles  except  turtles.  It  may  come  away  piecemeal,  or 
again,  as  in  snakes  and  lizards,  as  a  continuous  whole  ;  the  pro- 
cess of  separation  being  aided  by  the  formation  of  hair-like 
processes  developed  from  the  deeper  cells  which  lift  up  those 

1  Some  tropical  toads  have  bony  plates  beneath  the  skin  of  the  back. 


94        MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


portions  which  are  to  be  cast.  The  claws  of  the  reptiles  and 
the  horny  beaks  of  the  turtles  and  birds  are  also  cornifications 
of  the  epidermis. 

Scales  (upon  the  legs  and  feet)  and  claws  of  the  same  char- 
acter (/>.,  epidermal  cornifications)  reappear  in  the  birds,  but 
dermal  bones  are  never  found.  Birds  have  besides  a  peculiar 
epidermal  covering,  the  feathers,  which  must  have  more  atten- 
tion. In  the  following  account  it  must  be 
borne  in  mind  that  in  the  development  of 
the  feathers,  as  in  the  scale  of  a  snake,  a 
dermal  papilla  takes  the  initiative,  and  that 
the  ultimate  structure  is  entirely  formed  of 
cornified  epidermis. 

Feathers  occur  only  in  the  group  of 
birds,  and  here  three  principal  types  are 
found,  —  down-feathers,  pin-feathers,  and 
contour-feathers,  differing  much  in  ap- 
pearance, but  of  essentially  the  same  struc- 
ture. Contour-feathers  are  those  which 
cover  the  body  in  the  adult  bird,  giving  it 
its  outlines,  and  forming  the  broad  expanse 
of  wings  and  tail.  In  a  typical  contour- 
feather  are  to  be  distinguished  an  axial 
portion,  composed  of  a  proximal  hollow 
part,  the  quill,  and  a  distal  and  more  solid 
shaft,  the  latter  bearing  on  either  side 
lateral  outgrowths,  the  barbs ;  shaft  and 
barbs  making  up  the  vane.  Inside  the 
quill  occur  thin  structureless  partitions, 
the  pith,  while  the  shaft  bears  on  its  so- 
called  inferior  surface  a  longitudinal 
groove,  the  umbilicus.  The  barbs  bear 
on  their  sides  smaller  projections,  the  bar- 
bules,  which  are  usually  provided  with 
minute  hooks ;  these,  interlocking  with  similar  hooks  on  the 
adjacent  barbules,  convert  the  whole  vane  into  a  continuous 
sheet.  In  many  cases  a  second  or  aftershaft  joins  the  axis  of 
the  feather  on  the  lower  surface  near  the  Junction  of  the  main 


FIG.  100.  Contour 
feather,  a,  quill ;  b,  shaft 
c*  barbs. 


EPIDERMAL    STRUCTURES. 


95 


FiG.  101.  Part  of  a  feather,  enlarged.  ar 
portion  of  shaft  showing  a  part  of  a  barb  with 
its  barbules ;  b,  two  barbules  greatly  enlarged. 


shaft  with  the  quill.     The  vane  supported  by  this  after  shaft  is 
usually  more  downy  than 
the  others. 

Down-feathers  differ 
from  contour-feathers  in 
the  absence  of  a  shaft, 
the  barbs  arising  directly 
from  the  end  of  the  quill ; 
these  barbs  never  inter- 
lock, but  remain  soft  and 
:  -ee  from  each  other.  In 
pin-feathers  (filoplumes) 
there  is  merely  the  devel- 
opment of  a  hair-like  shaft 
without  barbs. 

Except    in    the    pen- 
guins and  some  ratite  birds,  feathers  are  not  uniformly  distrib- 
uted over  the  whole  surface  of  the  body,  but  occur  in  well-marked 

feather-tracts  or  pterylae,  the  rest  of 
the  surface  (apteria)  being  sparsely 
covered  with  down-  or  pin-feathers. 
The  tertiary  penguins  also  possessed 
feather-tracts,  so  that  their  absence  in; 
existing  penguins  must  be  a  secondary 
character.  The  arrangement  of  the 
feather-tracts  is  of  importance  in  the 
classification  of  birds. 

In  development  down-feathers  pre- 
cede contour-feathers.  There  first 
appears  in  each  spot  where  a  down 
feather  is  to  develop  a  rapid  multipli- 
cation of  dermal  cells,  thus  producing 
a  rudimentary  papilla,  over  which  the 
epidermis,  elsewhere  consisting  of 
Feather  tracts  in  basa]  ]ayer  and  epitrichium,  becomes 
several  cells  in  thickness.  By  contin- 
uous growth  the  papilla  becomes  long 
and  cylindrical,  projecting  from  the  body,  the  axial  derma  form- 


FlG.  102. 

young  of  common  crow  (  Corvus 
americanus}. 


96       MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


ing  the  pulp  of  the  future  quill,  while  the  epidermis  surrounds  the 
outgrowth.  A  circular  depression  around  the  base  of  the  papilla 
is  the  beginning  of  the  formation  of  the  future  feather  follicle. 
In  the  distal  portions  of  this  outgrowth  there  next  appear  longi- 


FIG.  103.  Two  stages  in  the  development  of  a  feather,  after  Davies.  b,  basal 
layer  of  epidermis;  d,  derma;  e,  epitrichium  ;  p,  pulp;  jc,  beginning  of  depression 
for  feather  follicle. 

tudinal  ridges  of  the  pulp  which  gradually  encroach  upon  the 
epidermis,  dividing  this  layer  into  a  series  of  cylindrical  rods 
(Fig.  104),  which  at  last  are  held  in  position  by  only  the  layer 
of  epitrichium.  Now  the  derma  retracts  into  the  feather  follicle, 

carrying  with  it  the  basal  layer  of 
the  epidermis,  so  that  there  re- 
mains a  hollow  epidermal  out- 
growth, the  quill,  bearing  at  its 
extremity  a  number  of  epidermal 
rods.  The  cells  of  these  portions 
rapidly  dry  and  become  cornified, 
and,  the  epitrichium  breaking 
away,  the  rods  separate  as  the 
down  of  the  down-feather. 

Later  the  contour-feathers  are 
developed  from  the  retracted  pulp 
which  grows  out  again  as  before. 
In  general  these  develop  like 
their  predecessors,  excepting  in 
certain  details.  The  rods  of  pulp 
are  not  longitudinal,  but  oblique 
to  the  axis  of  the  outgrowth,  the 
result  being  that  the  cornified  rods  (which  form  the  barbs) 
proceed  from  an  undivided  portion  (shaft)  on  the  dorsal  side  of 
the  outgrowth  ;  and  when  the  epitrichium  breaks  away,  these 


FIG.  104.  Transverse  section  of 
developing  down-feather  of  tern 
{Sterna  wilsoni}.  t>,  basal  layer  of 
epidermis;  bv,  blood-vessels;  e,  epi- 
trichium ;  ep,  epidermis  ;  /,  pulp ;  r, 
ridges  of  pulp  extending  into  epi- 
dermis. 


EPIDERMAL    STRUCTURES. 


97 


expand  so  as  to  form  the  vane.  One  point  needs  a  little  more 
detail.  On  that  side  where  the  shaft  is  to  be  formed  are  two 
longitudinal  thickenings  (Fig.  105)  ;  with  growth  these  become 
larger  and  bend  inwards  to  meet  each  other.  Near  the  tip  the 
result  is  a  solid  rod  (Fig.  105,  A),  but  farther  down  the  ingrowth 


FIG.  105.  Diagrammatic  sections  through  a  developing  contour-feather :  A  at 
about  the  middle  of  the  vane,  B  near  the  base  of  the  vane,  and  C  through  the  quill, 
after  Davies.  bt  barbs  ;  fs,  feather  sheath ;  /*,  /2,  different  portions  of  the  pulp 
cavity. 

includes  a  space  (Fig.  105,  B  and  C),  so  that  the  proximal  por- 
tion of  the  shaft  is  hollow.  The  umbilicus  is  formed  by  these 
ingrowing  ridges.  As  will  readily  be  understood,  the  so-called 
dorsal  and  ventral  sides  of  the  feather  correspond  to  the  outer 
and  inner  surfaces  of  the  epidermis  of  the  feather  papilla. 

At  regular  intervals  the  bird  sheds  or  molts  its  feathers,  the 
old   ones  dropping   out,   while  new    ones 
arise  to  take  their  place  by  a  repetition 
of  the  process  just  described. 

Hair  is  as  characteristic  of  mammals 
as  feathers  are  of  birds.  In  its  formation 
the  epidermis  apparently  takes  the  initi- 
ative, the  result  being  the  formation  of  a 
solid  ingrowth  of  epidermis  into  the  un- 
derlying  derma,  the  deeper  end  of  which  .  F/G'  I0?'  7™  stage* 

J  in    the   early  development 

becomes  cupped  (Fig.  106)  to  accommo-  of  the  hair  of  the  mouse, 
date  a  small  collection  of  dermal  cells, 
the  rudiment  of  the  hair  papilla.  Next  a 
circular  depression  appears  in  their  in- 
growth, separating  a  central  portion,  the 
future  hair,  from  the  surrounding  epidermis  which  forms  the 


after  Maurer.  D,  derma  ; 
£,  epithelial  hair-forming 
cells ;  F,  follicle  ;  /,  hair 
papilla. 


98        MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


hair  follicle.  In  both  hair  and  follicle  several  layers  may  be 
distinguished.  In  the  follicle  there  is  the  basal  layer  and  the 
more  superficial  layers  of  the  epidermis,  without,  however,  any 
clear  differentiation  of  strata  corneum  and  lucidum.  At  the 

bottom  of  the  follicle  (root  of 
the  hair)  these  pass  directly 
into  the  hair  itself,  on  the  out- 
side of  which  is  the  so-called 
inner  root-sheath  (the  walls  of 
the  follicle  forming  the  outer 
root-sheath)  composed  of  two 
layers  of  cells  (the  outer  called 
Henle's  layer  ;  the  inner,  Hux- 
ley's layer).  This  inner  root- 
sheath  does  not  reach  the 
external  surface.  The  hair 
proper  consists  of  a  central  core 
or  medulla,  around  which  are 
several  layers  of  cells,  the  cor- 
tex, and  on  the  outside  is  a  cutic- 
ular  layer.  The  growing  point 
is  at  the  bottom  of  the  follicle, 
where  the  basal  layer  of  the 
epidermis,  by  repeated  cell  di- 
vision, adds  to  the  base  of  the 
hair.  As  the  cells  grow  older 
they  become  cornified,  and  the 
whole  is  gradually  pushed  out  of  the  follicle  by  additions  below. 

Like  feathers,  hairs  appear  at  first  in  well-defined  tracts^; 
but  later,  by  multiplication,  this  regularity  is  lost.  Hairs  are 
least  abundant  in  the  whales,  where  they  may  be  reduced  to  from 
two  to  eight  pairs  in  the  neighborhood  of  the  mouth,  and  even 
these  sometimes  only  occur  during  fcetal  life.  Hairs  may  also 
be  enormously  developed  into  organs  of  defence,  as  in  the  case 
of  the  '  quills '  of  the  hedgehog  and  porcupines,  while  in  the 
case  of  the  vibrissae  ('whiskers')  near  the  mouth,  they  may 
serve  as  sense  organs  (p.  69). 

Oil  glands  of  the  racemose  type  are  usually  found  connected 


FIG.  107.  Diagrammatic  section  of 
a  hair  and  its  follicle,  after  Maurer. 
C,  cuticle ;  CR,  cortex ;  £,  epidermis  ; 
F,  follicle;  G,  oil  gland;  HE,  Henle's 
layer ;  HD,  Huxley's  layer  ;  M,  me- 
dulla; N,  nerves;  P,  hair  papilla;  S, 
outer  root  sheath  (the  inner  root  sheath 
is  composed  of  Henle's  and  Huxley's 
layers)  ;  V,  vein. 


EPIDERMAL    STRUCTURES. 


99 


with  the  hair  follicles,  while  a  system  of  smooth  muscle  fibres 
(especially  strong  in  porcupines)  serves  to  erect  the  hairs. 

Closely  related  to  hair  are  the  nails, 
claws,  and  hoofs  of  mammals,  and  the 
horn  of  sheep,  goats,  and  cattle ;  in  fact, 
these  structures  may  be  regarded  as  com- 
posed of  agglutinated  hairs. 

Somewhat  different  in  character  are 
the  scales  which  cover  the  body  in  the 
pangolins  (manids),  and  are  found  on  the 
tail  of  the  rodent  Anomalurus,  although 
these  are  both  of  epidermal  origin.  True 
dermal  bones  in  the  skin  occur  only  in 
the  armadillos  among  recent  forms,  where 
they  form  an  armor  upon  the  dorsal  sur- 
face of  the  body.  In  the  fossil  glypto- 
dons  the  body  was  enclosed  in  a  similar 
bony  case,  while  some  extinct  cetacea  possessed  dermal  bones. 


FIG.  108.  Hair  tracts 
on  early  cat  embryos,  after 
Maurer. 


IOO     MORPHOLOGY  OF  THE   ORGANS   OF   VERTEBRATES. 


MESOTHELIAL    STRUCTURES. 


THE  mesothelial  structures,  as  we  left  them  on  a  preceding 
page  (p.  8),  consisted  of  a  pair  of  compressed  sacs  or  pouches, 

one  on  either  side  of 
the  entodermal  tube. 
Each  pouch  consists  of 
an  inner  or  visceral, 
and  an  outer  or  soma- 
tic, wall,  the  cavity  be- 
tween them  being  the 
primitive  ccelom,  which 
is  now  entirely  cut  off 
from  all  other  cavities. 

FIG.   109.     Transverse   section    of   Amblystoma  ^,        ,      .        .  ,.     .  . 

embryo  after  the  separation  of  the   mesothelium. 

a,  archenteron;  c,  ccelom;  m,  mesothelial  walls  of  mesothelial     tissue    are 

coelom;    n,    notochord  ;    r,   groove    of   closure    of  now     to     be    described 

neural  tube;  s,  spinal  cord:  /,  canal  of  spinal  cord;  , 

olk  but  it  must  be  kept 

in  mind  that  there  are 

many  exceptions  to  the  details  as  given  below.  The  state- 
ments regarding  the  somites  apply  most  nearly  to  the  elasmo- 
branchs,  but  they  are  generalized  in  many  respects. 

First  in  order  is  the  development  of  the  primitive  segments 
or  somites  of  the  body.  It  is  to  be  noted  that  while  other  parts 
are  segmentally  or  metamerically  arranged  (nerves,  blood-ves- 
sels, skeleton),  this  metamerism  primarily  arises  in  the  mesothe- 
lium, and  becomes  secondarily  impressed  upon  other  structures. 
The  process  of  somite  formation  is  best  seen  in  the  trunk 
region. 

As  a  result,  partly  of  the  change  in  the  shape  of  the  em- 
bryo caused  by  the  infolding  of  the  medullary  plate  (see  ner- 
vous system),  in  part  of  the  growth  of  the  mesothelium  itself, 
the  ccelomic  pouches  extend  upwards  from  their  primitive  posi- 


MESOTHELIAL    STRUCTURES.  IOI 

tion  along  either  side  of  the  notochord  and  the  central  nervous 
system,  while  below  the  pouches  grow  until  they  meet  in  the 
mid  ventral  line,  below  the  entoderm.  In  each  coelomic  pouch 
three  horizontal  zones  are  to  be  distinguished,  —  a  dorsal  muscle- 
plate  or  myotome  zone  (epimere),  a  ventral  lateral  plate  zone 
(hypomere),  and  between  these  a  much  narrower  middle  zone 
(mesomere). 


FIG.  no.  Diagram  of  the  mesothelial  pouch  and  the  beginning  of  segmenta- 
tion, based  upon  Atnblystoma.  a,  anus;  c,  coelom;  e,  epimere;  h,  hypomere  ;  hp, 
hypophysis ;  ;//,  mesomere ;  mt,  mouth ;  »,  notochord ;  o,  eye ;  s,  spinal  cord. 

By  a  series  of  constrictions  not  easily  described,  but  readily 
made  out  from  the  figure,  epimere  and  mesomere  become  divided 
transversely  to  the  body  axis  into  a  series  of  cubical  bodies,  the 
protovertebrae  of  older  authors,  the  myotomes  of  recent  embry- 
ology. These  divisions  do  not  extend  into  the  hypomere,  and 
so  do  not  divide  the  ventral  part  of  the  coelom.  As  a  result  we 
have  below  a  single  coelomic  space  extending  the  length  of  the 
trunk,  which  connects  with  a  number  of  dorsal  coelomic  diver- 
ticula,  extending,  one  into  each  myotome.  Later,  horizontal  con- 
strictions cut  the  epimeral  portions  off  from  the  rest,  so  that  from 
this  region  there  arises,  on  either  side  of  the  body,  a  series  of 
completely  closed  cavities  with  epithelial  walls,  —  the  myotomes. 
To  avoid  confusion  with  that  portion  of  the  primitive  body  cavity 
(metacoele  or  splanchnocoele)  which  remains  between  the  lateral 
plates,  and  to  which  the  term  coelom  as  usually  applied  is  given, 
the  coelomic  pouches  in  the  myotomes  have  been  called  the 
myocoeles.  The  myotomes  give  rise  to  the  voluntary  muscles 
of  the  body  in  a  manner  shortly  to  be  described ;  the  modifi- 


102     MORPHOLOGY  OF  THE   ORGANS   OF   VERTEBRATES. 

cations  in  other  parts  of  the  coelomic  wall  must  be  outlined 
now. 

As  will  be  seen  by  the  diagram  above  (Fig.  1 10),  the  middle 
zone  becomes  segmented  at  the  same  time  and  in  the  same  way 
as  the  myotomes,  and  when  the  latter  cut  loose  from  the  rest  of 
the  mesothelial  tissue,  the  contained  coelom  becomes  roofed  in 
above.  From  the  inner  or  visceral  wall  of  these  mesomeric 
segments  there  is  now  a  rapid  proliferation  of  cells  upon  the 


FIG.  in.  Diagram  of  a  part  of  the  trunk  of  an  embryonic  vertebrate  showing 
the  development  of  the  mesothelial  tissues,  a,  aorta;  g,  neural  crest  (anlage  of 
spinal  ganglion);  i,  intestine;  n,  notochord;  p,  rudiment  of  pronephric  tubule;  s, 
sclerotome;  sc,  spinal  cord;  so,  somatic  layer  of  mesothelium;  sp,  splanchnic  layer. 
(For  later  conditions  compare  with  Fig.  127.) 

deeper  surface,  the  products  of  which  migrate  inward  around 
the  notochord,  where  they  eventually  give  rise  to  the  skeletal 
structures  (vertebrae)  surrounding  the  notochord  and  central 
nervous  system,  from  which  fact  these  immigrant  cells,  divided 
at  first  into  segments  like  the  coelomic  walls  which  gave  them 
origin,  are  called  sclerotomes.  From  the  method  of  formation  — 
budding  of  separate  cells  instead  of  an  involution  of  epithelial 
tissue  —  these  sclerotomes  must  be  regarded  as  mesenchymatous 
in  nature,  and  their  future  fate  must  be  described  in  connection 
with  that  layer.  It  is  only  necessary  to  say  here  that  these 


MESOTHELIAL    STRUCTURES, 


103 


g 


cells  are  not  wholly  used  up  in  building  the  solid  skeleton,  but 
that  some  wander  in  between  splanchnic  mesothelium  and 
entoderm,  where  they  give  rise  to  the 
smooth  muscles  and  connective  tissue 
of  the  alimentary  canal,  some  pass  be- 
tween the  myotomes,  where  they  form 
the  partitions  (myocommata  * )  be- 
tween these  structures,  while  others 
press  farther  and  give  rise,  in  part,  to 
the  deeper  layers  (cutis)  of  the  skin, 
etc.  This  same  middle  zone  also 
gives  rise  to  a  part  of  the  excretory 
system  (nephridia),  which  is  also  pri- 
marily divided  into  segments  (ne- 
phrotomes). 

The  lateral  plate  region  (hypo- 
mere)  shows  but  slight  traces  of 
segmentation.  From  the  dorsal  por- 
tion of  its  splanchnic  wall  arises  the 
glomus  of  the  pronephros  (see  below) 
and  the  gonads  (reproductive  struc- 
tures), but  whether  or  not  these  lat- 
ter are  truly  segmented,  and  whether 
we  have  metamerically  repeated  go- 
notomes,  is  as  yet  a  disputed  question. 
The  account  of  these  reproductive 
and  excretory  organs  will  be  given 
later. 

/  Mesenteries.  —  The  greater  por- 
tion of  the  lateral  plates  develops 
into  the  flattened  epithelium  (peri- 
toneum) lining  the  body  cavity 
(splanchnoccele  or  metacoele),  and 
plays  an  important  part  in  the  devel- 
opment of  the  walls  of  the  alimentary 
tract  and  the  membranous  supports 


FIG.  112.  Transverse  sec- 
tion of  embryo  dogfish  {Acan- 
thias}.  a,  aorta;  c,  coelom;  ^, 
ectoderm ;  g,  ganglion  of  spinal 
nerve;  A,  hypochorda;  /,  intes- 
tine; my  mesomere  ;  me,  mesen- 
chyme ;  ms,  mesentery ;  myt 
myotome;-  n,  notochord  ;  /, 
pronephric  duct ;  s,  sclerotome ; 
sc,  spinal  cord ;  so  and  J/, 
somatic  and  splanchnic  layers; 
sn,  spinal  nerve  ;  TV,  wall  of  in- 
testine. The  section  passes  on 
the  left  side  through  the  middle 
of  a  myotome,  on  the  right  near 
the  edge  of  one. 


1  The  term  myocomma  is  sometimes  regarded  as  a  synonym  of  myotome ;  the  usage 
adopted  here  is  preferable ;  myoseptum  is  another  term  for  it. 


104     MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


(mesenteries,  in  the  broader  sense  of  the  word)  which  connect 
the  various  viscera  to  the  walls  of  the  body  cavity.  The  condi- 
tions in  the  abdominal  region  will  be  described  first. 

Here  the  splanchnic  layer  of  the  mesothelium  applies  itself 

to  either  side  of  the 
walls  of  the  alimen- 
tary tract,  it  being 
of  course  kept  in 
mind  that  mesenchy- 
matous  tissue  has 
migrated  in  between 
entoderm  and  meso- 
thelium in  this  re- 
gion (see  p.  103), 
while  above  and  be- 
low the  digestive 
tract  the  dorsal  and 
ventral  walls  of  the 
hypomere  press  in- 
wards towards  the 
median  line,  insinu- 
ating themselves 
dorsally  between  the 
alimentary  canal  and 


FIG.  113.  Diagrammatic  section  of  vertebrate 
through  abdominal  region,  a,  dorsal  aorta;  c,  ccelom; 
g,  gonad;  gl,  glomerulus;  i,  alimentary  canal;  /,  trally  between  the 
liver;  ///,  mesentery ;  mu,  muscular  layer  of  myotome;  entocjerm  ancj  ecto. 
my,  myocoele  ;  n,  nephrostome ;  na,  neural  arch ;  nc, 
notochord  ;  o,  omentum ;  s,  spinal  cord ;  so,  somatic 
layer  of  peritoneum;  sp,  splanchnic  layer  of  peri- 


the    notOCnOrd, 


derm.      As    a   result 
there    is    formed    a 


toneum  ;  /,  nephridial  tubule;  vmt  ventral  mesentery  ;  Double  partition  be- 
w,  Wolffian  duct. 

tween  the  metacoeles 

of  the  two  sides  both  above  and  below  the  intestine,  with  a 
small  amount  of  mesenchymatous  tissue  between  the  two  epi- 
thelial walls.  These  partitions,  which  thus  come  to  support 
the  alimentary  canal  (Fig.  1  1  3),  are  the  dorsal  and  ventral 
mesenteries. 

The   ventral    mesentery   is  never   perfect    throughout    the 
abdominal  cavity.     In  the  posterior  portion  the  partition  walls 


MESOTHELIAL    STRUCTURES. 


105 


break  down,  placing  the  coeloms  of  the  two  sides  in  free  com- 
munication. In  front  a  part  of  this  ventral  mesentery  persists, 
binding  the  liver  to  the  anterior  abdominal  wall,  and  in  many 
ichthyopsida  carrying  the  sub-intestinal  vein  to  that  organ. 
Another  portion,  known  as  the  small  omentum  (or  gastro-hepatic) 
and  the  duodeno-hepatic  omentum  extends  from  the  dorsal 
surface  of  the  liver  to  the  stomach  and  duodenum  (Fig.  113). 
The  dorsal  mesentery  is  usually  far  more  complete.1  In  it 
are  recognized  various  regions,  named  according  to  the  organs 
which  they  support,  —  mesogaster,  mesentery  proper,  mesocolon, 


FIG.  114.  Three  stages  in  the  development  of  the  alimentary  canal  and  the 
mesenteries  of  man,  after  Toldt  and  Hertwig.  a,  appendix  vermiformis ;  ao,  aorta; 
b,  bile  duct  ;  c,  csecum ;  co,  colon ;  d,  duodenum ;  go,  great  omentum ;  me,  meso- 
colon ;  me,  mesentery ;  mg,  mesogaster  ;  /,  pancreas ;  r,  rectum  ;  s;  stomach  ;  si, 
small  intestine  ;  sp,  spleen  ;  tc,  transverse  colon ;  v,  vitelline  duct.  The  arrow 
points  to  the  opening  of  the  omentum. 

mesorectum,  etc.  It  is  attached  to  the  dorsal  wall  in  a  straight 
line,  and  in  those  vertebrates  with  a  straight  alimentary  canal 
the  mesentery  is  a  plane  membrane,  but  with  increasing  con- 
volution of  the  alimentary  canal,  the  membrane  becomes  corre- 
spondingly plaited.  Besides  this  complication,  the  mesenteries 
can  form  secondary  unions  with  the  body  wall,  or  with  the 

1  In  Petromyzon  (cyclostome)  it  has  entirely  disappeared,  except  a  few  shreds  in  the 
rectal  region. 


106     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

mesenterial  regions,  the  details  of  which  must  be  sought  in 
special  works. 

Those  mesenterial  folds  which  bind  the  various  regions  of 
the  alimentary  canal  to  each  other  have  received  the  special 
name  of  omenta.  The  small  omentum  has  just  been  men- 
tioned ;  the  gastro-splenic  omentum  connects  the  spleen  with 
the  stomach  ;  while  in  the  higher  vertebrates  the  great  omen- 
tum is  a  large  double  fold  formed  from  the  mesogaster  and 
the  mesocolon,  which  connects  the  stomach  to  the  transverse 
colon. 

In  the  region  of  the  heart  and  of  the  lungs  (when  these 
organs  are  present)  the  splanchnic  layer  of  the  ccelomic  wall 
becomes  similarly  related  to  these  structures,  and  in  a  similar 
way  similar  supporting  folds  (mediastinum  for  the  lungs,  meso- 
cardium  for  the  heart)  are  formed.  In  the  abdominal  region 
similar  mesothelial  folds  (mesorchium  in  the  male,  mesovarium  or 
mesoarium  in  the  female)  support  the  reproductive  organs 
(gonads). 

Divisions  of  Splanchnocoele.  —  So  far,  that  part  of  the  ccelom 
enclosed  between  the  lateral  plates  has  been  considered  as  a 
single  space  on  either  side,  as  it  is  in  the  early  development. 
Soon,  however,  the  anterior  portion  becomes  cut  off  from  the 
rest  and  forms  a  sac,  the  pericardium,  enclosing  the  heart,  the 
relations  of  which  are  described  in  connection  with  the  circu- 
latory organs.  In  the  lower  vertebrates  the  posterior  wall  of 
this  pericardium  is  known  as  the  false  diaphragm  or  septum 
transversum,  and  in  many  is  perforated  by  one  or  more  small 
pericardio-peritoneal  canals,  connecting  the  pericardium  with  the 
abdominal  cavity,  a  result  of  incomplete  closure. 

In  the  mammals  the  true  diaphragm  appears,  although  rudi- 
ments, sometimes  even  muscular,  appear  in  some  sauropsida. 
This  diaphragm  is  a  transverse  muscle,  usually  described  as 
crossing  the  abdominal  cavity  from  side  to  side,  completely 
dividing  it  into  two  cavities,  an  anterior  or  pleural,  in  which 
the  lungs  are  placed,  and  a  posterior  or  peritoneal  cavity  con- 
taining the  remaining  viscera.  This  statement  is  not  exactly 
correct.  In  the  lower  forms  the  liver  abuts  directly  against 
the  septum  transversum.  In  the  mammals  these  relations  are 


MUSCULAR  SYSTEM. 


ID/ 


the  same.  The  diaphragm  is  therefore  to  be  regarded  as  a 
paired  structure,  extending  from  the  lateral  walls  behind  the 
lungs  to  middle  part  of  the  septum  transversum.  This  explains 
why  it  is  that  the  pericardium  appears  as  if  enclosed  in  the 
pleural  cavity,  although  it  is  morphologically  outside  of  it. 


FIG.  115.  Diagrams  illustrating  the  relations  of  the  pericardium  to  the  rest 
of  the  coelom:  A  in  fishes,  B  in  amphibia  and  sauropsida,  C  in  mammalia,  d,  dia- 
phragm; //,  heart;  /,  liver;  /,  lungs;  s,  septum  (false  diaphragm)  between  peri- 
cardium and  the  rest  of  the  ccelom.  In  B  the  lungs  project  into  the  general  ccelom 
(pleuro-peritoneal  cavity),  in  C,  by  the  formation  of  the  diaphragm,  pleural  and 
peritoneal  cavities  are  distinct,  while  the  pericardial  cavity,  containing  the  heart,  has 
been  shoved  backwards  between  the  two  pleural  cavities. 

The  abdominal  coelom  is  not  completely  closed  off  from  the 
outer  world  ;  for  the  urogenital  ducts,  to  be  described  later,  form 
a  means  of  communication.  Besides  these  there  occur  in  cyclos- 
tomes,  many  fishes,  dipnoi,  turtles,  and  crocodiles,  from  one  to 
two  small  openings  (known  as  abdominal  pores),  beside  or  be- 
hind the  vent,  by  means  of  which  the  coelom  is  connected  with 
the  outside  world.  Little  is  known  as  to  their  function. 


MUSCULAR    SYSTEM. 

The  history  of  the  muscle  plates  or  myotomes  is  next  to  be 
taken  up.  After  their  separation  from  the  other  portions  of  the 
primitive  mesothelial  tissue,  these  form  a  series  of  approximately 


IO8         MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


cubical  hollow  bodies  on  either  side  of  the  notochord  and  cen- 
tral nervous  system.  From  the  early  idea  that  these  bodies 
gave  rise  to  the  vertebrae,  they  were  formerly  called  proto- 
vertebrae.  We  now  know  that  they  contribute  little  ^or  noth- 
ing to  the  skeletal  structures,  but  give  rise  to  the  voluntary 
musculature  of  the  body.  The  processes  involved  in  the  con- 
version of  these  epithelial  walls  into  muscle  —  the  histogenesis 
of  muscle  —  must  be  traced  first. 

In  the  majority  of  the  vertebrates  the  cells  of  the  mesal 
wall  (i.e.,  that  towards  the  notochord  and  nervous  system)  rap- 
idly increase  in  number,  thus  obliterating 
the  myoccele.  In  this  process  the  cells  lose 
their  original  shape  and  arrangement  as  a 
cylindrical  epithelium,  and  form  elongated 
cylinders,  the  axes  of  which  are  parallel  to 
the  longitudinal  axis  of  the  body.  Each  of 
these  primitive  muscle  cells  at  first  contains 
but  a  single  nucleus  ;  but  by  division  several 
arise,  which  may  either  eventually  lie  in 
the  centre  (amphibia)  or  on  the  periphery 
(mammals)  of  the  cell.  At  the  same  time 
the  peripheral  protoplasm  of  the  cell  be- 
comes differentiated  into  numbers  of  fine 
longitudinal  fibrillae,  which  increase  in  num- 
ber so  that  at  last  all  except  a  small  amount 
of  protoplasm  in  the  immediate  vicinity  of 
the  nucleus  has  been  converted  into  these 
contractile  structures,  —  the  epithelial  cell 
becomes  a  muscle  fibre.  The  lateral  or 
outer  wall  of  the  myotome  does  not  par- 
ticipate in  this  muscle  formation,  but  is  said 
to  give  rise  to  the  deeper  layer  (corium  or 
derma)  of  the  skin.  The  process  of  the  histo- 
genesis of  muscle  in  the  cyclostomes  differs 
in  some  particulars  from  that  given  above. 
The  myotomes,  after  their  separation  from  the  mesothelial 
tissues,  increase  rapidly  in  their  dorso-ventral  dimensions,  and 
gradually  push  in  between  the  lateral  plate  and  the  ectoderm  in 


FIG.   116.     Myo- 
tomes of  Amblystoma 

in  process  of  conver- 
sion intomuscle-plates. 
c,  remains  of  myo- 
coele;  ck,  chorda;  ^, 
epidermis;  ;;/,  muscle 
developing  from  inner 
plate  of  myotome  ;  o, 
outer  plate  of  myo- 
tome ;  s,  skeletogenous 
tissue. 


MUSCULAR  SYSTEM. 


IC>9 


the  ventral  half  of  the  body,  thus  giving  rise  to  the  musculature 
of  this  region,  while  dorsally  their  extension  is  less  marked 
(Fig.  1 1 3).  In  this  process  the  myocommata  also  participate,  so 

that  the  whole  body  is  enveloped  on 
either  side  by  a  series  of  muscle-plates, 
the  fibres  of  which  have  a  generally 
longitudinal  direction,  and  are  inter- 
rupted at  regular  intervals  by  the  in- 
termuscular  ligaments,  the  derivatives 
of  the  earlier  myocommata.  This 
primitive  condition  can  readily  be  rec- 
ognized in  the  trunk  region  of  a  fish, 
but  it  becomes  greatly  modified  in  the 

FIG.  117.  Illustrating  the 
downward  growth  of  the 
myotomes.  e,  epimere  (myo- 
tome)  ;  m,  mesomere. 

birds  and  mammals  ;  yet 
even  here  traces  of  the  primi- 
tively segmented  condition 
can  be  made  out  in  the 
ventral  abdominal  region  and 
in  the  intercostal  muscles. 

In  the  fishes  the  result- 
ing muscles  of  the  trunk  and 
tail  become  subsequently  di- 
vided into  dorsal  and  ventral 
or  epiaxial  and  hypaxial  sys- 
tems, the  line  of  division 
between  the  two  following 
more  or  less  closely  the  lat- 
eral line,  and  being  marked 
by  a  partition  of  connective 
tissue.  In  the  amphibia 
these  epi-  and  hypaxial  por- 
tions are  clearly  visible  in  the  tail,  but  farther  forward  the 
hypaxial  system  is  reduced.  This  reduction  is  carried  to  a 
greater  extent  in  the  aminotes,  where  almost  the  sole  traces 


FIG.  118.  Transverse  section  through 
young  Aniblystoma,  showing  the  final  re- 
sult of  the  downward  growth  of  the  myo- 
tomes (deeply  shaded)  ;  a,  aorta  ;  c,  car- 
dinal vein;  /,  liver;  o,  oesophagus;  /, 
peritoneum ;  pg,  pectoral  girdle ;  r,  rib  ; 
s,  subvertebral  vein. 


I  10      MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


of  the  hypaxial  system  are  to  be  found,  greatly  modified,  in  the 
pelvic  and  neck  regions. 

The  subsequent  modifications  of  the  primitive  musculature 
in  the  higher  groups  cannot  be  traced  here  in  detail,  even  were 
it  better  known.  Only  the  origin  of  the  limb  muscles  can  be 
referred  to.  This  is  best  known  in  the  fishes,  there  being  only 


FIG.  119.  Section 
through  the  tail  of  Am- 
blystoma,  showing  (<?) 
epiaxial  and  (///)  hypaxial 
muscles;  h,  haemal  arch; 
»,  notochord;  r,  ribs. 


FIG.  1 20.     Developing  fin  of  trout,  after  Corning. 
fin;  m,  myotomes;   n,  notochord.     Strands  of  cells  can 
be  traced  from  several  myotomes  into  the  fin. 


scattering  observations  relating  to  the  air-breathing  vertebrates  ; 
but  these  few  accounts  justify  us  in  the  assumption  that  in  the 
amniotes  the  phenomena  are  essentially  the  same.  In  the  fish- 
like  forms  several  of  the  somites  almost  immediately  behind 
the  head  bud  from  their  lower  surfaces  cords  of  cells  which 
extend  out  laterally,  lose  their  distinctness,  and  form  a  common 
matrix  out  of  which  the  definitive  muscles  are  later  developed 
(Fig.  1 20).  In  the  amniotes  more  somites  intervene  between  the 


MUSCULAR  SYSTEM. 


Ill 


head  and  those  myotomes  which  form  the  muscles  of  the  limb. 
The  process  for  the  formation  of  the  posterior  fin  is  essentially 
the  same. 

In  the  head  region,  although  no  little  study  has  been  de- 
voted to  the  subject  of  the  mesoderm  segments,  naturalists  are 
not  in  unison  as  to  their  results.  Not 
only  is  there  a  difference  of  opinion  as  to 
the  number  of  myotomes  developed  in 
this  region,  from  the  nine  recognized  by 
van  Wijhe  to  the  seventeen  or  more 
claimed  by  Dohrn  and  Killian,  but  it  is 
even  disputed  whether  these  be  true 
somites.  The  questions  involved  will  be 
taken  up.  in  another  place.  Setting  these 
points  aside,  it  may  briefly  be  said  that 
in  those  forms  which  have  been  most 
carefully  studied  (the  elasmobranchs) 
there  are  ten  l  myotomes  developed  in  the 
head  region.  Each  of  these  which  occur 
in  front  of  the  ear  is  completely  separated 
from  its  fellows,  a  fact  which  leads  some 
to  believe  that  we  have  to  do  here,  not 
with  the  whole  mesothelial  structures  as 
in  the  trunk  region,  but  with  merely  the 
myotome  zone.  This  matter,  however, 
would  seem  to  have  less  importance  than 
is  sometimes  given  to  it ;  for  we  must  re- 
member that  the  vertebrate  head  is  an 


FIG.  121.  Section 
through  the  head  of  em- 
bryo Acanthias  at  about  the 
stage  of  Fig.  122.  a,  aoita; 
ac,  anterior  (premandibu- 
lar)  head  cavity;  e,  pig- 
mented  epithelial  layer  of 


*'  notochord  J  A  pharynx; 

retinal 


eye;   /,     fore    brain; 

extremely  complicated  structure,  all  the     Gasserian    ganglion; 

parts  of   which    have    been    greatly    modi-      hind  brain  ;  /,  lens  of  eye  ; 

fied,  while  the  appearance  of  the  gill  slits 

,  ..   ,,     —  .  i  •    At     u      > 

is  of  itself  sufficient  to  explain  the   ab-     head  cavities. 
sence  of  a  continuous  metaccele. 

To  the  muscle  fibres,  the  development  of  which  was  out- 
lined above,  other  parts  of  mesenchymatous  origin  are  added 
in  the  development  of  the  definitive  muscle.  This  connective 

1  There  is  clearly  one  pair  of  coelomic  cavities  in  front  of  the  first  recognized  by  van 
Wijhe  (Fig.  121,  ac). 


112       MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


tissue  wanders  in  among  the  separate  muscle  cells,  forms  an 
envelope  (perimysium),  and  binds  these  together  into  bundles, 
and  bundles  into  muscles.  This  perimysium,  which  extends 
beyond  the  contractile  or  true  muscular  portions,  also  forms 
the  means  of  attachment l  of  the  muscles  to  the  parts  which 
are  to  be  moved ;  it  gives  rise  to  what  are  known  as  ten- 
dons, sometimes,  as  in  the  extremities,  of  considerable  length. 
Tendons  may  occur  not  only  at  the  ends,  but  in  the  middle 
of  muscular  tracts.  When  forming  broad,  flat  sheets,  tendons 

my  v          gl        o        f        g     t 


FIG.  122.  Anterior  end  of  embryo  dogfish,  Acanthias,  viewed  as  a  trans- 
parent object,  a,  anterior  head  cavity;  b,  first  true  gill  cleft;  e,  eye;  f,  facial 
nerve ;  f(>,  fore  brain ;  g,  Gasserian  ganglion ;  gl,  glossopharyngeal  nerve ;  h,  heart ; 
m,  position  of  mouth;  nib,  mid  brain;  my,  myotome ;  o,  auditory  capsule;  /, 
pinealis ;  s,  spiracular  cleft;  /,  trigeminal  nerve;  v,  vagus  nerve;  I,  2,  first  and 
second  head  cavities  of  van  Wijhe. 

are  called  fascia  or  aponeuroses.  Frequently  ossification  may 
occur  in  tendons,  familiar  examples  being  found  in  the  patella 
or  knee-pan  of  man,  the  bony  tendons  in  the  '  drumstick '  of 
many  birds  (turkeys),  etc. 

In  shape  the  muscles  vary  extremely.  In  the  trunk  region, 
as  a  rule,  they  are  short  and  more  or  less  flattened  ;  in  the  ex- 
tremities they  are  usually  prismatic  or  cylindrical,  and  greatly 
elongate.  They  may  have  one  or  several  « heads '  or  points  of 

1  That  attachment  of  a  muscle  which  usually  remains  without  motion  in  the  contrac- 
tion of  a  muscle  is  spoken  of  as  its  origin  ;  the  attachment  to  a  movable  portion  as  its 
insertion. 


MUSCULAR  SYSTEM. 


origin  (biceps,  triceps,  etc.)  ; 
one  or  several  points  of  inser- 
tion (pinnate,  bipinnate,  ser- 
rate, etc.). 

In  the  fish-like  vertebrates 
the  trunk  muscles  clearly  show 
their  myotomic  origin,  for  myo- 
tomes  and  the  intervening  myo- 
commata  are  strikingly  in  evi- 
dence. Even  here  there  is  a 
tendency  toward  specialization, 
for  a  horizontal  connective  tis- 
sue partition  divides  the  muscles 
of  each  half  of  the  body  into 
dorsal  and  ventral  portions  (p. 
109)  ;  while  in  the  ventral  region 
occurs  a  subdivision  into  a  me- 
dian rectus  muscle,  and  a  more 
lateral  oblique  muscle  (the 
names  being  indicative  of  the 
direction  of  the  muscle  fibres). 
These  same  features  can  be 
traced  more  or  less  clearly  in 
the  higher  vertebrates,  compli- 
cations being  introduced  by  the 
greater  development  of  those 
muscles  which,  while  having 
their  origin  on  the  trunk,  serve 
to  move  the  limbs,  and  by  the 
subdivision  of  the  others,  into 
distinct  regions.  .  Thus  the  rec- 
tus may  be  divided  into  a  rectus  FIG.  123. 
abdominis,  extending  from  the  °f  Necturus. 

ceps ;  C,  cora 
cobrachialis ; 

ceratohyoid  ;  EO,  external  oblique;  F,  flexor  communis;  FC, 
gracilis;  Gff,  geniohyoid;  1C,  ileocaudalis ;  LA,  linea  alba; 
mylohyoid  ;  MO,  middle  oblique ;  P,  pectoralis  ;  PC,  procoracoid ; 
Pff,  procoracohumeralis ;  PT,  pectineus ;  PY,  pyriformis  ;  KA 
jRI,  rectus  internus ;  SC,  supracoracoid ;  SH,  sternohyoid ;  VI, 


Ventral  muscles 
A,  anus  ;  B,  bi- 
coid ;  CB,  cora- 
ECH,  external 
f emorocaudalis ;  G 
If,  masseter;  MH, 
PF,  pubofemoralis ; 
,  rectus  abdominis ; 
vastus  internus. 


114     MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


pelvis  to  the  breast-bone,  a  sterno-hyoid  from  the  sternum  to 
the  hyoid  region,  and  a  genio-hyoid  from  the  hyoid  to  the 
extremity  of  the  lower  jaw.  Similarly  the  oblique  muscles  may 
be  subdivided  into  three  or  more  layers  (internal  and  external 
oblique,  transverse,  etc.);  intercostals,  between  the  ribs;  scalenes, 
from  the  anterior  ribs  to  the  side  of  the  neck,  and  sterno-  and 
cleido-mastoid  from  the  breast-bone  and  clavicle  to  the  skull. 
In  the  dorsal  half  of  the  trunk  also  a  large  number  of  separate 
muscles  may  be  distinguished,  —  spinales,  between  the  spinal 
processes  of  the  vertebrae ;  inter-transversales,  between  the 
transverse  processes  ;  longissimus  dorsi,  arising  from  the  ribs 
and  transverse  processes,  and  extending  along  the  back  (con- 
tinued in  the  cranial  region  as  the  trachelo-mastoid)  ;  recti  capi- 
tis,  etc.  The  muscles  of  the  diaphragm  are  indirectly  derived 
from  the  ventral  portion  of  the  myotome. 

In  the  gill  region  of  the  branchiate  vertebrates  special  mus- 
cles are  developed  from  the  corresponding  myotomes  to  open 
(levator  and  depressor  arcuum)  and  to  close  (constrictors)  the 
gill  slits.  With  the  loss  of  the  gills  these  muscles  change  in 

their  functions,  and  become  connected 
with  the  hyoid  or  disappear.  The  jaws 
are  opened  by  a  digastric  muscle  arising 
from  the  base  of  the  skull,  and  inserted 
on  the  angle  of  the  jaw,  while  closure 
of  the  mouth  is  effected  by  adductors, 
called  masseter,  temporalis,  or  pterygoid, 
according  to  their  origin  from  different 
regions  of  the  skull. 

The  muscles  which  move  the  ball  of 
the  eye  are,  in  all  vertebrates,  six  in 
number,  and  are  derived  from  the  three 
anterior  head  somites  of  van  Wijhe  (p. 
in).  The  most  anterior  of  these  devel- 
ops into  three  rectus  muscles,  —  supe- 
rior, internal,  and  inferior,  and  into  the 
inferior  oblique;  the  second  furnishes 
the  superior  oblique,  and  the  third  the  external  rectus.  It  is 
interesting  to  note  that  the  nerve  supply  of  these  muscles  corre- 


FIG.  124.  Eye  muscles 
and  nerves  in  shark,  a, 
abducens;  otn,  oculomotor; 
/,  trochlearis  nerves ;  oi,  os, 
inferior  and  superior  oblique 
muscles ;  re,  ri,  rif,  rs,  ex- 
ternal, internal,  inferior,  and 
superior  rectus  muscles. 


MUSCULAR  SYSTEM.  115 

spends  to  this  origin  (see  cranial  nerves).  These  muscles  move 
the  eye,  and  in  many  forms  are  re-enforced  by  a  retractor  bulbi, 
apparently  derived  from  the  third  head  segment. 

Beneath  the  skin  of  mammals  there  occurs  a  general  mus- 
cular layer,  the  panniculus  carnosus,  concerning  which  our 
information  is  none  too  extensive.  From  this  layer  are  devel- 
oped in  the  facial  region  '  muscles  of  expression,'  which  serve 
to  move  the  skin,  especially  that  around  the  mouth  and  eyes. 
The  fact  that  these  muscles  of  expression  are  innervated  by  the 
facial  nerve  would  apparently  indicate  their  point  of  origin  as 
behind  the  jaws. 

The  muscles  which  move  the  limbs  are  divided  into  intrinsic 
(those  which  have  their  origin  and  insertion  on-  the  bones  of 
the  limb  or  of  the  supporting  girdle)  and  extrinsic  (which  arise 
from  the  trunk  and  are  inserted  on  the  girdle  or  on  the  limb). 
In  the  fishes  neither  series  acquires  extensive  development ;  but 
with  the  more  varied  movements  necessary  in  a  terrestrial  life 
both  sets,  and  especially  the  intrinsic,  attain  a  high  grade  of 
differentiation.  Both  series  may  be  grouped  as  dorsal  and  ven- 
tral, and  these  divisions  again  may  be  considered  accordingly  as 
they  are  preaxial  or  postaxial  in  position  ;  i.e.,  accordingly  as 
they  are  in  front  of  or  behind  the  axis  of  the  limb.  The  proxi- 
mal extrinsic  and  intrinsic  preaxial  muscles  act  as  protractors, 
serving  to  move  the  limb  forwards,  the  postaxial  as  retractors, 
which  move  it  in  the  opposite  direction.  The  other  intrinsic 
muscles  are  divided  between  flexors,  which  bend  the  limb  upon 
itself,  and  extensors,  which  straighten  it  after  flexion.  For 
the  details  of  these  muscles  reference  must  be  made  to  special 
works. 

Eletrical  Organs.  —  In  certain  fishes,  Torpedo,  electrical  eel 
(Gymnotus),  Malapteru$,  and  to  a  less  degree  in  some  skates 
{Raia),  certain  of  the  muscles  become  metamorphosed  into  an 
electrical  organ.  This  organ  lies  in  the  Torpedo  on  either  side 
of  the  head  ;  in  the  others  in  the  trunk  or  tail  near  the  back- 
bone. In  all  the  organ  consists  of  a  series  of  capsules  of  con- 
nective tissue  filled  with  a  gelatinous  substance  in  which  are 
the  'electrical  plates,'  in  which  the  nerves  terminate,  and  which 
are  apparently  the  modified  motor  end-plates  of  the  muscle. 


Il6     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

The  discharge  of  the  organ  is  under  control  of  the  will,  and 
varies  in  strength  according  to  the  size  of  the  organ  and  its  con- 
dition of  fatigue  ;  in  the  torpedo 
and  electrical  eel  it  is  sufficient  to 
knock  a  man  down,  but  in  the 
others  it  is  much  less  in  amount. 

UROGENITAL    ORGANS. 

The  excretory  and  tne  repro- 
ductive organs  of  the  vertebrates 
are  so  closely  related  to  each  other 
that  it  is  impossible  to  treat  them 
separately.  The  excretory  glands 
(nephridia),  reproductive  organs 
proper  (gonads),  and  the  ducts  to 
carry  away  the  nitrogenous  waste 
and  the  reproductive  elements, 
stand  in  close  relation  to  each 
other  in  position,  development, 
and  function.  Hence  We  speak  of 
urogenital  organs. 

Nephridia.  —  Under  the  head  of  nephridia  are  to  be  included 
three  different  structures  which  appear  in  the  vertebrates,  —  a 
pronephros  (  '  head  kidney  '  of  older  writers)  ;  a  mesonephros  or 
Wolffian  body ;  and  a  metanephros,  which  is  the  functional  kid- 
ney in  the  amniotes.  Pronephros  and  mesonephros  appear  only 
as  embryonic  structures  in  the  amniotes  ;  but  in  the  lower  groups 
the  pronephros  is  usually  functional  for  a  time,  the  mesone- 
phros assuming  its  work  in  the  adult.1  These  organs  have  a 
regular  succession  in  time,  and  hence  in  our  account  we  follow 
the  order  of  development  and  begin  with  the  pronephros. 

It  will  be  remembered  (p.  101)  that  the  walls  of  the  meso- 
thelial  cavities  on  either  side  are  divisible  into  three  zones,  and 
that  segmentation  only  affects  the  dorsal  and  middle  of  these, 
the  hypomere  being  unsegmented.  The  nephridial  structures 


FIG.  125.  Diagrammntic  sec- 
tion of  electrical  apparatus,  from 
Wieclersheim.  The  arrow  points 
dorsally  or  anteriorly.  BG,  con- 
nective tissue  framework ;  EPy 
electrical  plates;  G,  gelatinous  tis- 
sue ;  A7,  nerves  entering  through 
the  septa;  AW,  terminations  of 
the  nerves. 


i  In  the  elasmobranchs  the  pronephros  is  never  functional,  while  apparently  in  Bdel 
lostoma  (a  cyclostome)  the  whole  excretory  organ  is  pronephric. 


UKOGENITAL    ORGANS. 


117 


arise  almost  entirely  from  the  mesomeric  segments  (nephro- 
tomes).  The  pronephros  arises  from  a  few l  nephrotomes  im- 
mediately behind  the  head.  From  the  outer  wall  of  each  of  these 
an  outgrowth  occurs,  —  sometimes  solid  at  first,  but  usually 
hollow  from  the  beginning,  —  its  apex  directed  towards  the  skin. 
These  outgrowths  form  the  pronephric  tubules,  each  of  which 
opens  at  the  inner  end,  by  means  of  the  remains  of  the  cavity  of 
the  mesomere,  into  the  body  cavity,  the  opening  being  funnel- 
shaped  and,  in  its  full  development,  ciliated.  These  openings 


B 


FIG.  126.  Diagrams  of  the  relations  of  pro-  and  mesonephros,  based  on 
Semon.  Mesomeric  structures  shown  with  conventionalized  cells.  A,  pronephros  ; 
B,  mesonephros.  a,  aorta ;  b,  Bowmanjs  capsule  ;  d,  pronephric  duct ;  g,  gonad ; 
gl,  glomerulus;  gs,  glomus ;  in,  inner  nephrostome ;  ;;/,  myotome ;  ms,  formation  of 
mesenchyme ;  /«/,  mesonephric  tubule ;  n,  cavity  of  nephrotome ;  ns,  nephrostome  ; 
on,  outer  nephrostome  ;  /,  pronephric  tubule. 

are  the  nephrostomes.  Distally  the  tubules  of  the  successive 
segments  fuse  together,  thus  giving  rise  to  a  longitudinal  tube, 
the  pronephric  duct,  which  gradually  extends  backwards  be- 
hind the  pronephric  segments,  until  at  last  it  fuses  with  the 
cloaca  or  with  the  skin'  immediately  adjoining.  An  opening 
now  forms  between  the  duct  and  the  cloaca,  and  thus,  through 
the  system  of  tubes  leading  from  the  nephrostomes  to  the  vent, 
the  body  cavity  is  placed  in  connection  with  the  external  world. 

1  Two  in  most  urodeles  and  amniotes  ;  three  in  lampreys,  some  sharks,  anura,  and  some 
amniotes  ;  four  in  some  sharks,  seven  or  eight  in  skates,  and  a  dozen  in  caecilians.  It  has 
been  pointed  out  that  in  general  terms  the  number  of  pronephric  nephrotomes  is  roughly 
correlated  to  the  number  of  segments  in  the  whole  body. 


Il8      MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 

This  backward  growth  of  the  pronephric  duct  is  apparently  (at 
least  in  most  forms)  the  result  of  growth  of  the  duct  itself, 
without  any  cellular  additions  from  other  sources ;  although  a 
few  years  ago  the  duct  was  described  in  most  vertebrates  as 
being  wholly  of  ectodermal  origin,  a  view  arising  from  the  fact 
that  the  duct  in  its  progress  fuses  with  that  layer. 

A  second  pronephric  element  is  the  glomus.  The  dorsal 
arterial  blood-vessel  (aorta)  gives  off  an  arterial  twig  on  either 
side  opposite  each  nephrostome.  Each  artery  breaks  up  into  a 
vascular  network  just  beneath  the  dorsal  splanchnic  epithelium 
of  the  hypomere  (Fig.  126,  A),  and  pushes  out  so  that  the  struc- 
ture projects  into  the  dorsal  portion  of  the  body  cavity.  This 
vascular  outgrowth  is  the  glomus.  In  most  vertebrates  it  is 
unsegmented,  but  forms  a  continuous  rete  mirabile,  and  projects 
freely  into  the  coelom.  In  certain  forms,  however  (e.g.,  Ichthy- 
opJiis),  the  glomus  becomes  distinctly  segmented,  while  the 
dorsal  portion  of  the  body  cavity  becomes  cut  off  from  the  rest, 
forming  a  separate  envelope  (Bowman's  capsule)  around  each 
glomar  segment,  so  that  here  we  have  a  series  of  vascular  cap- 
sules almost  exactly  comparable  to  the  Malpighian  bodies  to  be 
described  in  connection  with  the  mesonephros. 

There  is  considerable  evidence  to  show  that  the  pronephros 
originally  had  a  much  greater  extent  than  in  most  existing 
forms ;  and  indeed  the  structure  may  have  extended  nearly  to 
the  vent,  as  is  apparently  the  case  in  Bdellostoma,  if  we  may 
judge  by  recent  studies.  Its  fate  in  all  vertebrates  except  this 
cyclostome  will  be  better  understood  after  a  history  of  the  meso- 
nephros. ' 

The  mesonephros  or  Wolffian  body  is  usually  confined  to 
segments  behind  the  pronephros,  and  is  often  spoken  of  as  a 
later  generation  of  excretory  structures.  The  fact,  however, 
that  pro-  and  mesonephric  tubules  can  occur  together  in  the 
same  segment  tends  to  show  that  the  two  structures  are  dis- 
tinct. 

The  mesonephric  tubules  are  formed  in  a  manner  similar  to 
the  pronephric  tubules,  except  that  they  arise  from  the  more 
dorsal  portion  of  the  nephrotome.  They  grow  outwards,  and 
finally  connect  with  the  pronephric  duct,  although  they  do  not 


UROGENITAL   ORGANS. 


119 


participate  in  its  formation.  From  this  time  on  the  prone- 
phric  duct  is  usually  called  the  mesonephric  or  Wolffian  duct. 
The  aorta  likewise  forms  segmental  twigs,  which  grow  out  to- 
wards the  splanchnic  layer  of  the  nephrotome,  and  give  rise 
to  a  series  of  vascular  networks,  the  glomeruli,  which  differ  from 
the  glomus  of  the  pronephros  in  that  they  project  not  into  the 
larger  body  cavity  (splanchnoccele),  but  into  the  cavity  of  the 


sm  a    cv 


FIG.  127.  Diagram  of  the  development  of  the  nephridial  system  in  the  verte- 
brate. The  pronephric  system  is  according  to  the  views  of  Semon  ;  it  is  more  prob- 
able that  they  arise  from  the  nephrotomes  instead  of  from  the  somatic  layer,  a, 
aorta;  c,  notochord ;  cv,  cardinal  vein;  cl,  pronephric  duct;  g,  glomus  •  £tf,  gonad; 
z,  intestinal  epithelium;  m,  myotome  (muscular  layer);  mb,  Malpighk  .  body;  me, 
myoccele ;  mt,  mesonephric  tubule ;  n,  nephrotome ;  ns,  nephrostome  ;  pt,  prone- 
phric tubule;  sc,  spinal  cord;  sg,  spinal  ganglion;  sm,  sympathetic  ganglion;  so, 
somatic  layer  ;  sp,  splanchnic  layer  ;  z,  cavity  of  nephrotome.  Compare  with  Fig.  1 1 1. 

nephrotome,  and  in  their  segmental  arrangement.  The  walls 
of  the  nephrotome  close  over  each  glomerulus,  and  are  hence- 
forth known  as  Bowman's  capsule,  while  the  whole  complex  of 
capsule  and  glomerulus  form  a  Malpighian  body  or  corpuscle. 
The  mesonephric  tubule  opens  into  Bowman's  capsule  by  means 
of  an  inner  nephrostome,  while  the  lower  portion  of  the  nephro- 


120     MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


tome  cavity  retains  its  connection  with  the  metacoele,  the  open- 
ing forming  the  outer  nephrostome  (Fig.  126,  j5). 

As  was  stated  above,  pro-  and  mesonephros  are  the  only 
excretory  organs  in  the  ichthyopsida.  They  appear  as  larval 
structures  in  the  amniotes,  and  only  in  certain  reptiles  does 
either  of  them  function  after  hatching.  In  the  adult  ichthyop- 
sidan  the  pro-  and  mesonephros 
are  usually  readily  distinguished, 
the  pronephros  being  in  front, 
rudimentary  in  character,  and 


\v 


FIG.  128.  A  single  tubule  of  the 
mesonephros  of  Proteus  anguineus 
modified  from  Spengel.  C,  begin- 
ning of  collecting  tubule ;  B,  Bow- 
man's capsule  ;  6",  glomerulus ;  /, 
ON,  inner  and  outer  nephros  - 
tomes. 


FIG.  129.  Section  through  the  meso- 
nephric  region  of  Amblystoma,  45  mm.  long. 
A,  aorta;  B,  Bowman's  capsule,  from  which 
the  glomerulus  has  dropped  out ;  C,  carti- 
lage, and  />,  bone  of  vertebral  centrum  ;  (7, 
gonad ;  GL,  glomerulus ;  M,  mesentery  ; 
MA,  mesenteric  artery.;  A7,  notochord ;  T, 
mesonephric  tubules;  W,  Wolffian  duct. 


separated  by  a  greater  or  less  distance  from  the  Wolffian  body, 
which  usually  extends  along  the  greater  part  of  the  dorsal  wall 
of  the  body  cavity. 

The  pronephros  acquires  a  varying  development  in  different 
vertebrates.  In  the  elasmobranchs  its  tubules  never  become 
convoluted,  and  in  very  early  embryonic  life  the  nephrostomes 
fuse  so  that  a  single  large  opening  connects  the  coelom  with  the 
pronephric  duct,  and  forms  the  anterior  end  of  the  Mullerian 


UROGENITAL    ORGANS.  121 

duct  to  be  described  below.  In  the  amniotes,  also,  the  pro- 
nephros  never  advances  beyond  a  very  rudimentary  condition, 
and  soon  degenerates,  and,  to  a  greater  or  less  extent,  disap- 
pears. In  ganoids,  teleosts,  and  amphibia  the  pronephros  is 
functional  for  a  time.  The  tubules  become  greatly  convoluted, 
and  between  them  is  developed  a  rich  plexus  of  sinus-like  blood- 
vessels. Later  it  degenerates  in  all  except  a  few  teleosts  (Fier- 
asfer,  Dactylopterus),  where  it  remains  functional  throughout  life, 
while  in  others  it  retains  its  excretory  character  until  the  ap- 
proach of  sexual  maturity.  In  these  teleosts  with  functional 
pronephros  the  funnels  connect  with  the  pericardial  cavity  (the 
same  condition  has  been  described  in  cyclostomes),  a  relation 
readily  understood  from  the  method  of  formation  of  the  pericar- 
dial walls.  In  its  degeneration  the  pronephros  contributes  to 
the  formation  of  the  supra-renal  bodies  to  be  described  below. 

In  its  development  the  mesonephros  progresses  beyond  the 
stage  at  which  it  was  left  above.  The  tubules,  instead  of  being 
short  and  transverse,  become  greatly  convoluted,  and  they  also 
increase  greatly  in  number,  new  tubules,  with  funnels  and  Mal- 
pighian  bodies,  being  developed  by  budding  dorsal  to  the  primary 
tubules  ;  and  after  a  convoluted  course  these  secondary  and  ter- 
tiary tubules  join  the  distal  ends  of  the  first,  which  thus  become 
converted  into  collecting  tubules,  emptying  into  the  pronephric 
duct.  With  this  formation  of  new  tubules  the  mesonephros 
largely  loses  the  segmental  character  that  it  earlier  possessed. 

With  the  convolution  and  increase  in  number  of  the  tubules 
blood-vessels  enter  between  these  structures,  and  form  a  rich 
capillary  plexus  surrounding  them.  The  cells  of  the  tubules 
become  cubical  and  excretory  in  character.  This  increase  in 
number  and  size  of  the  tubules  increases  the  size  of  the  organs, 
so  that  they  protrude  into  the  coelom  as  a  ridge  on  either  side 
of  the  mesentery. 

The  physiological  action  of  pro-  and  mesonephros  is  appar- 
ently as  follows  :  Blood  from  the  aorta  enters  the  glomus  or 
glomeruli,  through  the  walls  of  which  it  loses  water,  which  passes 
(pronephros)  into  the  ccelom  or  (mesonephros)  into  Bowman's 
capsule,  and  from  thence  into  the  tubules.  From  the  glomeruli 
the  blood  next  passes  into  the  plexus  surrounding  the  tubules, 


122     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


and  here,  by  means  of  the  cubical  cells,  loses  its  nitrogenous 
waste  (uric  acid,  urates,  etc.).  By  means  of  the  cilia  surround- 
ing the  nephrostomata,  watery  matter  is  also  taken  from  the  coe- 
lom,  and  all  of  these  waste  products  are  passed  via  the  pronephric 
duct  to  the  exterior. 

In  teleosts  and  ganoids  all  of  the  mesonephros  is  excretory ; 
but  in  elasmobranchs  and  amphibians  the  anterior  end  loses  this 
function  and  becomes  largely  degenerate  (females),  or  enters 
into  the  service  of  the  reproductive  structures  (males),  as  will 
be  described  below.  In  the  amniotes  the  whole  mesonephros 
degenerates  and  disappears,  except  in  so  far  as  it  enters  into 
connection  with  the  gonads,  and  is  represented  by  the  paradidy- 
mis  and  parovarium  (infra). 

To  compensate  for  this  disappearance  a  third  excretory 
organ,  the  metanephros,  or  kidney  proper,  is  developed  in  the  am- 
niotes. Its  developmental  history  is  not  so  well  known  as  that 
of  the  pro-  and  mesonephros,  and  the  following  statement  is  only 
tentative.  A  hollow  diverticulum  arises  from  the  dorsal  sur- 
face of  each  pronephric  duct,  near  its  entrance  into  the  cloaca. 

This  grows  rapidly  forward  near 
the  aorta,  and  develops  into  the 
excretory  duct  (ureter)  of  the 
metanephros.  As  it  grows  for- 
ward the  mesoderm  behind  the 
Wolffian  body  rapidly  proliferates, 
and  becomes  richly  vascular. 
When  the  ureter  reaches  the  hin- 
der end  of  the  Wolffian  body  it 
expands,  giving  rise  to  the  pelvis 
of  the  kidney,  and  produces,  by 
budding  from  its  tip,  cords  of  cells 
which  soon  become  tubular,  and 
form  the  collecting  tubules  of  the 
kidney.  In  the  proliferated  meso- 
derm other  tubules  also  appear 
(the  method  of  their  formation  is  not  clear)  connected  with 
Malpighian  bodies,  essentially  like  those  of  the  mesonephros. 
These  metanephric  tubules  become  greatly  convoluted,  and  at 


FlG.  130.  Kidneys  (/£)  and  supra- 
renals  (j)  of  a  human  embryo,  after 
Wiedersheim.  The  figure  shows  the 
lobulated  appearance  of  the  early 
kidney. 


UROGENITAL    ORGANS. 


123 


last  open  into  the  collecting  tubules ;  but  it  is  important  to 
note  that  at  no  time  are  nephrostomata  developed  in  connec- 
tion with  them,  and  the  body  cav- 
ity is  without  communication  with 
this  nephriclial  system.  While  this 
process  is  going  on  the  whole  met- 
anephros  pushes  farther  forward, 
dorsal  to  the  pronephric  duct,  the 
ureter  increasing  correspondingly 
in  length.  In  the  subsequent  his- 
tory the  kidney  becomes  strongly 
lobulated,  the  lobes  corresponding 
to  the  groups  of  collecting  tubules 
of  which  it  is  composed.  This  lob- 
ular  appearance  is  retained  through- 
out life  in  the  sauropsida,  but  is 
subsequently  lost  in  all  mammals 
except  the  whales  and  some  car- 
nivores. 

The  kidney  never  extends 
through  as  many  segments  as 
does  the  mesonephros,  but  forms  a 
relatively  smaller  and  more  com- 
pact body  lying  within  or  a  little 
in  front  of  the  pelvic  region.  In 
the  mammals  the  anterior  end  of 
the  ureter  becomes  widened  out, 
inside  of  the  mass  of  the  kidney, 
into  a  considerable  chamber,  the 
pelvis  of  the  kidney,  into  which 
the  collecting  tubules  empty,  the 
openings  of  these  being  placed  on 
one  or  more  papillae,  which  extend 
into  the  pelvis  renalis,  partially 
dividing  it  into  smaller  chambers 
or  calyces. 

The  ureter  does  not  long  retain  its  primary  connection  with 
the  distal  end  of  the  pronephric  duct,  but  acquires  its  own  open- 


FIG.  131.  Urogenital  system 
of  male  heron  (Ardea},  from 
Wiedersheim.  Ao,  aorta;  £F, 
bursa  Fabricii,  opening  at  BF'  into 
the  cloaca,  Cc;  Ep,  epididymis; 
Ho,  testes  ;  A7,  kidney ;  Sr,  opening 
of  ureter ;  V,  furrows  for  veins  on 
ventral  surface  of  kidney ;  Vd,  vasa 
deferentia;  Vd',  their  opening  into 
cloaca. 


124     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

ing  into  the  dorsal  portion  of  the  cloaca ;  and  then  this  cloacal 
region  becomes  constricted  off  from  the  rest  to  form  a  urinary 
bladder,  which  is  connected  directly  or  indirectly  with  the  ex- 
terior by  a  single  duct,  —  the  urethra.  The  bladder  persists 
throughout  life  in  lizards,  turtles,  and  mammals,  but  disappears 
in  the  other  amniotes. 

Reproductive  Organs.  —  To  those  structures  which  are  to 
produce  the  reproductive  cells,  —  eggs  and  spermatozoa,  —  the 
term  gonads  has  been  given.  These  are  paired  (unless  fusion  or 


IG.  132.  Section  of  ovary  of  new-born  child,  from  Hertwig  after  Waldeyer. 
/,  single  egg  surrounded  by  follicle  cells ;  g,  group  of  egg  cells  and  follicle  cells ; 
ge,  germinal  epithelium ;  /,  egg  strings  ;  /<?,  primordial  ova ;  v,  blood-vessels. 

abortion  of  one  occur),  and  arise  from  the  epithelium  lining 
the  body  cavity,  nearer  the  median  line  than  the  nephrosto- 
mata  (see  Fig.  127,  gcTy.  In  this  region,  which  may  extend 
nearly  the  length  of  the  body  cavity  or  which  may  be  more 
restricted,  the  epithelium  retains  its  original  columnar  character, 
and  even  increases  it,  while  in  all  other  regions  it  becomes 
converted  into  a  pavement  epithelium.  The  underlying  mesen- 
chyme  increases  in  amount,  pushing  the  germinal  epithelium  out 
into  the  body  cavity  as  a  longitudinal  ridge.  It  is  usually 
stated  that  in  the  earlier  stages  the  gonads  are  segmental  in 


UROGENITAL   ORGANS. 


12$ 


character  ;  i.e.,  are  divided  into  segments,  which,  in  harmony 
with  the  names  given  other  regions,  have  been  called  gonotomes, 
but  the  accuracy  of  these  statements  has  been  disputed  recently. 
In  the  earlier  stages  certain  of  the  epithelial  cells  become 
larger  than  their  fellows ;  and  these  are  called  the  primordial 
ova  and  the  primordial  seminal  cells,  accordingly  as  they  are  to 
give  rise  to  eggs  or  spermatozoa.  For  a  considerable  length 
of  time  one  cannot  say,  from  an  examination  of  these  primor- 
dial cells,  whether  they  are  to  develop  into  one  or  the  other  of 
these  reproductive  cells ;  but  other  structures  enable  us  to 
decide  at  an  early  date,  in  most  vertebrates,  whether  we  have 
to  deal  with  a  male  or  a  female,  and  so  between  testes  and 
ovaries. 

The  ovaries  are  those  gonads  which  are  to  give  rise  to  the 
eggs  or  ova.  Briefly  stated,  portions  of  the  epithelium  cover- 
ing the  gonad  sink  into 


the  deeper  portion  of 
the  ovary,  carrying  with 
them  the  primordial  ova, 
while  the  other  cells  ar- 
range themselves  as  fol- 
licles around  these  ova. 
In  this  position  the  ova 
increase  rapidly  in  size, 
in  part  by  what  must 
be  called  a  devouring  of 
their  neighbors,  in  part 
by  nourishment  fur- 
nished through  the  fol- 
licular  cells  from  the 


FlG.  133.  Portion  of  ovary  of  cat.  b,  blood- 
vessel ;  d,  discus  proligerus ;  <?,  nearly  mature 
ovum ;  /,  follicle  epithelium ;  o,  clusters  of  im- 
mature ova. 


richly  vascular  mesenchyme  adjacent.  In  the  mammals  these 
follicles  undergo  a  peculiar  modification,  and  have  secured  the 
special  name  of  Graafian  follicles.  The  follicular  epithelium  be- 
comes several  cells  in  thickness  and  then  splits,  thus  forming  an 
internal  cavity,  filled  with  fluid,  to  one  side  of  which  the  ovum, 
surrounded  by  a  few  follicular  cells,  remains  attached  (Fig.  I  33). 
This  region  is  the  discus  proligerus.  When  fully  formed  the 
follicle  rises  to  the  surface  of  the  ovary,  and  at  the  proper  time, 


126      MORPHOLOGY  OF   THE  ORGANS   OF   VERTEBRATES. 


by  rupture  of  the  surrounding  walls,  the  egg  escapes.      In  the 
higher  vertebrates   usually  but  a  single  egg  escapes  at  a  time, 
but  in  the   ichthyopsida  hundreds  or  even  thou- 
o(]\          sands  may  pass  out  from  the  ovary  in  a  few  hours. 
As  will  be  understood  from  the  relations  of  the 
ovary,  the  eggs  upon  their  escape  pass  into  the 
coelomic  cavity,  from  which  they  pass  to  the  ex- 
terior, in  the  case  of  some  fishes,  by  means  of  the 
abdominal  pores,  but  in  most  vertebrates  through 
the  Miillerian  duct. 

At  first  the  ovaries  are  simple  ridges  project- 
ing slightly  into  the  coelorn,  but  as  they  increase 
in  size  they  become  more  distinct  from  the  body 
wall,  until  at  last  they  are  only  supported  by  a 
double  fold  of  the  peritoneum,  like  a  mesentery, 
the  mesovarium. 

The  male  gonads  which  produce  the  sperma- 
tozoa are  called  testes,  and,  like  the  ovaries,  their 
essential  portions  are  derived  from  the  germinal 
epithelium.  This  in  places  sinks  into  the  under- 
lying mesenchyme  as  cords  of  cells.  Later  these 
cords  become  hollow  (canaliculi  seminiferi),  and 
from  certain  large  round  cells  in  their  walls,  the 
spermatozoa  are  formed  by  cell  division.  In  the 
case  of  the  testes  there  is  the  same  formation  of  a 
mesenterial  support  (mesorchium)  as  was  noted  for 
the  ovary. 

The  Urogenital  Ducts.  —  In  teleosts,  ganoids, 
and  cyclostomes  the  pronephric  duct  remains  with- 
out essential  alteration,  functioning  solely  as  an 
excretory  duct.  In  the  elasmobranchs  it  divides 
into  two  tubes.  In  the  mesonephric  region  its 
r^stiuntmbi  lumen  becomes  oval  in  section,  the  collecting  tu- 
bules emptying  into  the  dorsal  portion.  It  now 
divides  lengthwise  back  as  far  as  the  cloaca,  while 
in  front  the  division  stops  just  in  front  of  the 
mesonephros  (Fig.  134).  The  dorsal  of  the  two  resulting  tubes 
is  the  Wolffian  (Leydig's)  duct,  the  ventral  the  Miillerian  duct. 


FIG.  134. 

Division  of 
pronephric 
duct  into  Miil- 
lerian (»/)  and 
Wolffian  O) 
ducts  in  elas- 
mobranchs; /, 
funnels;  g  , 
glomeruli  of 


formed  by  fu- 
sion of  prone- 
phric funnels. 


UROGENITAL    ORGANS.  12 / 

• 

The  former  retains  its  connection  with  the  menesophros ;  but 
the  Mlillerian  duct  loses  all  connection  with  the  Wolffian  body, 
and  opens  into  the  coelom  by  means  of  the  fused  nephrostomes 
of  the  pronephros,  —  the  ostium  tubae  abdominale. 

In  all  other  vertebrates  a  Miillerian  duct  is  formed,  ac- 
cording to  the  older  accounts  in  the  same  way  as  in  the  elas- 
mobranchs,  but  according  to  most  recent  writers  as  a  new 
formation.  An  ingrowth  of  coelomic  epithelium  begins  at  the 
anterior  end  of  the  mesonephros,  and  continues  backward  until 
,the  cloaca  is  reached.  During  its  growth  it  becomes  tubular, 
and  at  its  anterior  end  it  opens  widely  into  the  coelom,  behind 
into  the  cloaca.  It  thus  forms  a  Miillerian  duct  essentially  like 
that  of  elasmobranchs  in  structure,  but  greatly  different  in 
origin.  In  all  cases  the  duct  receives,  in  addition  to  the  epithe- 
lium, mesenchymatous  tissue,  which  makes  up  the  bulk  of  its 
walls. 

The  Miillerian  duct  in  the  female  of  both  elasmobranchs 
and  of  higher  vertebrates  henceforth  functions  as  an  oviduct. 
The  eggs,  which  escape  from  the  ovary,  pass  into  its  funnel 
(ostium),  and  are  thence  conducted  to  the  exterior.  It  may 
remain  a  simple  tube,  or  more  usually  distinct  regions  may  be 
specialized  in  it,  each  with  distinct  functions.  Most  constant 
of  these,  except  in  mammals,1  are  regions  which  secrete  the 
protective  envelopes  —  shell,  egg  membrane,  etc.  —  around  the 
egg.  In  those  forms  which  bring  forth  living  young  (mammals, 
many  elasmobranchs,  etc.)  one  portion  of  the  duct  becomes 
specialized  to  retain  the  egg  during  its  development,  and  re- 
ceives the  name  uterus  in  those  forms  where  the  growing  young 
acquires  attachment  to  the  lining  walls.  With  the  develop- 
ment of  the  uterus  that  portion  of  the  Miillerian  duct  in  front 
is  called  the  Fallopian  tube,  while  the  post-uterine  portion  forms 
a  vagina.  In  the  lower  mammals  as  in  the  lower  vertebrates 
the  Miillerian  ducts  usually  remain  distinct  from  each  other 
throughout  their  length,  the  result  being  two  uteri  and  two  va- 
ginae ;  in  the  higher  mammals  a  fusion  of  the  posterior  end  of 
the  ducts  of  the  two  sides  occurs,  resulting  in  a  single  vagina, 
and  usually  of  an  impaired  uterus,  the  latter  showing  more  or 

1  The  monotremes  differ  from  the  other  mammals  in  this  respect. 


128      MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


less   clearly  traces  of  its  double   origin,  as  in   cases  of  bicor- 
nuate  uteri. 

In  the  male  the  Mullerian  duct  almost  entirely  disappears, 
a  portion  of  its  anterior  end  persisting  as  the  stalked  hydatid 
(or  hydatid  of  Morgnani),  somewhat  closely  connected  with  the 

V«*^       5%Z,t,;n 

0ln      frr 

\  1      r=*Hn  !   /jL^S 


FIG.  135.  Diagram  of  the  modifications  in  the  urogenital  apparatus.  A,  in- 
different and  also  the  female  ichthyopsidan ;  £,  male  amphibian ;  C,  male  amniote ; 
Z>,  female  amniote.  c,  cloaca;  e,  epididymis;  /,  fimbriated  extremity  of  Fallopian 
tube;  g,  gonad;  h,  stalked  hydatid;  k,  kidney  (metanephros)  ;  m,  Mullerian  duct; 
mn,  mesonephros  (Wolffian  body);  o,  ovary;  od,  oviduct;  ot,  ostium  tuBae;  pd, 
paradidymis;  po,  paroophoron ;  pv,  parovarium ;  r,  rete ;  /,  testes;  «,  uterus;  urn, 
uterus  masculinus;  ur,  ureter;  va,  vas  aberrans;  vd,  vas  deferens;  ve,  vasa  effer- 
entia;  ?v,  Wolffian  duct. 

epididymis  (see  below),   while    the  posterior  end  occasionally 
retains  its  lumen,  and  is  known  as  the  uterus  masculinus. 

The  history  of  the  Wolffian  duct  is  somewhat  different.  In 
the  female  its  anterior  end  degenerates  ;  and  in  the  amniotes, 
where  the  metanephros  usurps  the  functions  of  the  mesone- 
phros, this  degeneration  extends  to  the  whole  tube.  The  only 
portions  which  persist  are,  a  rudimentary  structure  behind 


UROGENITAL    ORGANS. 


I29 


known  as  Gartner's  duct,  and  in  front  where  it  comes  in  con- 
nection with  a  small  body  known  as  the  parovarium  or  epooph- 
oron1  formed  from  the  degenerate 
tubules  of  the  mesonephros. 

J>  "  The    Wolffian     duct     persists 

'  throughout  life  in  the  male,  where 


-Of 


Htt- 


m/(&f) 

A  B 

FIG.  136.  Scheme  of  urodele  urogenital  system  based  on  Triton,  from  Wied- 
ersheim  after  Spengel.  A,  male;  B,  female,  a,  excretory  ducts;  GN,  sexual 
part  of  mesonephros;  Ho,  testis;  lgt  Leydig's  duct  (ureter);  mg,  Mullerian  duct 
(oviduct  in  j?) ;  mg' ',  its  vestigial  end  in  the  male ;  A7,  functional  portion  of  meso- 
nephros;  Ov,  ovary;  Of,  ostium  tubse;  Ve,  vasa  efferentia  ;  -f-  collecting  duct  of  the 
vasa  efferentia  (rudimentary  in\5). 

it    acquires    new  functions.      Here    the    anterior    end    of    the 
mesonephros  loses  its  excretory  powers,  and  enters  into  con- 

1  In  amniotes,  where  the  whole  mesonephros  degenerates,  the  posterior  portion  of  the 
Wolffian  body  in  the  female  forms  a  parobphoron  behind  the  ovary,  a  structure  of  only 
vestigial  importance. 


130      MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


nection  with  the  testis.  Its  tubules  branch  and  anastomose, 
and  also  connect  with  the  seminiferous  canaliculi,  forming  a 
system  of  ducts  conducting  the  spermatozoa  into  the  anterior 
end  of  the  Wolffian  duct.  This  plexus  of  tubules  nearest  the 
testis  is  the  rete,  nearer  the  duct  it  forms  the  vasa  efferentia. 
The  Wolffian  duct,  by  this  assumption  of  reproductive  func- 
tions, is  converted  into  a  vas  deferens,  the  anterior  part  of 

which  becomes  greatly  coiled,  this  por- 
tion being  known  as  the  epididymis. 
More  distally  the  duct  may  develop 
marked  muscular  walls  and  form  an 
ejaculatory  structure. 

As  was  said  above,  the  posterior 
portion  of  the  mesonephros  retains  its 
excretory  powers  in  the  ichthyopsida, 
and  as  it  pours  its  secretions  into  the 
Wolffian  duct,  this  tube  in  the  male  is 
at  once  excretory  and  reproductive  in 
character.  In  the  male  amniotes,  when 
the  metanephros  is  developed,  the  pos- 
terior part  of  the  mesonephros  degen- 
erates into  a  small  body  close  to  the 
epididymis  known  as  the  paradidymis 
(organ  of  Giraldi),  and  occasionally 
forms  one  or  more  blind  tubes  (vasa 
aberrantia),  opening  into  the  vas 
deferens. 

In  some  vertebrates  the  Wolffian 
and  Miillerian  ducts  open  directly  into 

the  cloaca,  but  in  most  they  unite  into  a  urogenital  sinus,  which, 
in  turn,  empties  by  a  single  opening  into  the  cloaca.  In  the 
mammals  (monotremes  excepted)  the  separation  of  the  uro- 
genital sinus  from  the  hinder  end  of  the  alimentary  canal  is 
complete,  a  muscular  partition  (perineum)  separating  the  vent 
from  the  urogenital  opening. 

Connected  with  the  urogenital  structures  outlined  above  are 
many  accessory  parts,  —  glands,  external  genitalia,  copulatory 
organs,  etc., — some  of  which  will  be  described  in  connection 


FIG.  137.  Urogenital  or- 
gans of  male  frog;  the  testis, 
T,  turned  to  one  side  to  ex- 
pose A,  the  vasa  efferentia 
passing  from  the  testis  to  M, 
the  mesonephros.  F,  poste- 
rior end  of  fat  body ;  P,  post- 
cava;  S,  suprarenal;  Uy 
ureter. 


UROGENITAL   ORGANS.  131 

with  the  groups  in  which  they  occur.  Others  will  be  found  in 
the  larger  manuals  to  which  reference  must  be  made.  Only 
one  of  these  structures  seems  to  demand  attention  here.  This 
is  the  suprarenal  body,  which  derives  its  name  from  the  fact 
that  in  the  mammals  it  forms  a  capsule-like  structure  on  the 
anterior  end  of  the  kidney.  In  the  sauropsida  it  is  in  closer 
connection  with  the  gonads.  In  the  amphibia  (Fig.  137)  it  is 
either  on  the  ventral  surface  of  the  mesonephros  (anura),  or 
upon  its  medial  margin  (urodeles).  In  the  teleosts  it  is  either 
closely  connected  with  the  mesonephros,  or  is  farther  forward 
in  the  region  of  the  degenerate  pronephros.  In  the  elasmo- 
branchs  the  suprarenal  is  replaced  by  two  structures:  (i),  an 
interrenal,  a  long,  slender  body  just  medial  to  the  ureter,  those 
of  the  two  sides  being  connected  behind ;  (2),  a  series  of 
adrenals,  on  either  side  closely  connected  with  the  sympathetic 
ganglia. 

Development  teaches  that  the  suprarenals  consist  of  two 
portions  different  in  origin  and  corresponding  to  the  inter-  and 
adrenals  of  the  elasmobranchs.  One  portion  (the  cortical  sub- 
stance, interrenals)  arises  from  the  mesothelium,  and  according 
to  recent  observations,  largely  by  a  metamorphosis  of  the  glomus 
of  the  pronephros,  the  mesonephros  possibly  contributing  to 
some  extent.  The  medullary  substance  (equivalent  to  the  ad- 
renals) is  derived  in  part  from  the  sympathetic  nervous  system, 
and  contains  ganglion  cells,  and  in  part  is  mesenchymatous  in 
nature,  this  tissue  arising  from  cells  proliferated  by  the  septum 
transversum. 


132     MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


MESENCHYMATOUS  STRUCTURES. 

As  was  stated  on  a  preceding  page  (p.  8),  the  mesenchyme 
may  arise  from  ectoderm,  entoderm,  or  mesothelium,  either  by 
the  separation  of  isolated  cells,  as  is  usually  the  case,  or  by  the 

immigration  of  large  masses  of 
cells  into  the  space  (i.e.,  the  re- 
mains of  the  segmentation  cavity) 
between  the  other  body  layers. 
This  immigration  in  large  masses 
from  the  mesothelium  is  shown  in 
the  formation  of  the  sclerotomes 
in  Fig.  1 1 2,  and  from  the  ectoderm 
into  the  region  of  the  head  to  form 
the  gill  cartilages  in  Fig.  138.  The 
mesenchyme  is  characterized  by 
the  fact  that  it  never  gives  rise  to 
epithelial  structures,1  and  as  a  rule, 
by  the  great  development  in  it  of 
intercellular  substance,  as  seen  in 
fibrous  or  areolar  connective  tissue, 
cartilage,  bone,  blood,  etc.  Smooth 
muscle  tissue,  however,  is  an  ex- 
ception in  this  respect. 

Besides  the  connective  tissues 
proper,  which  extend  through  all 
parts  of  the  body,  forming  a  sup- 
port and  connection  for  tissues  and 
organs,  the  mesenchyme  also  gives 
rise  to  most  of  the  skeletal  and  circulatory  structures. 


FIG.  138.  Section  through  the 
head  of  an  embryo  Amblystoma, 
showing  the  points,  H  and  Aft 
where  the  ectoderm  is  producing 
the  mesenchyme  to  form  the  hyoid 
and  mandibular  arches.  A,  audi- 
tory ganglion  ;  C,  ccelom  of  man- 
dibular arch ;  CL,  cuticular  layer 
of  ectoderm ;  MO,  medulla  ob- 
longata ;  NL,  nervous  layer  of 
ectoderm;  VII,  facial  nerve. 


1  It  is  possible  that  the  epithelium  (endothelium)  lining  the  cavities  of  the  vascular 
system  is  of  mesenchymatous  origin,  but  the  weight  of  evidence  goes  to  show  that  some  of 
it  at  least  is  of  entodermic  origin. 


SKELETON.  133 


THE    SKELETON. 

The  skeletal  structures  of  the  vertebrates  may  be  either 
membranous,  cartilaginous,  or  bony  (osseous)  in  character ;  and 
in  development  certain  portions  may  pass  successively  through 
all  of  these  phases  in  attaining  the  adult  condition ;  or  the 
cartilage  stage  may  be  skipped,  the  membrane  developing 
directly  into  bone ;  or  again,  the  cartilaginous  condition  may 
be  the  final  stage  of  the  skeleton. 

The  membranous  skeleton  consists  of  connective  tissue 
cells,  and  in  its  highest  development  forms  sheets  or  masses 
of  fibrous  tissue.  From  it  cartilage  is  developed  by  a  great 
increase  in  the  number  of  cells,  the  tissue  in  what  has  been 
called  the  procartilage  stage  consisting  of  closely  compacted 
polygonal  cells  with  large  nuclei.  These  cells  rapidly  secrete 
an  intercellular  substance  (chondrin),  and  thus  the  tissue  be- 
comes converted  into  cartilage,  the  extent  and  solidity  of  which 
are  dependent  upon  the  amount  of  this  matrix.  In  the  conver- 
sion of  cartilage  into  bone  this  matrix  is  dissolved  ;  and  around 
the  margins  of  the  cavities  thus  produced  bone-forming  cells 
(osteoblasts)  arrange  themselves,  and  these,  secreting  lime  salts 
(carbonate  and  phosphate)  around  themselves,  gradually  build 
up  the  bone.  In  the  lower  vertebrates  this  process  begins 
upon  the  outside  of  the  cartilages  and  proceeds  toward  the 
interior ;  but  in  the  higher  forms,  besides  this  perichondrial 
ossification,  centres  of  ossification  appear  within  the  cartilage, 
and  from  these  the  ossification  extends  peripherally.  In  the 
conversion  of  membrane  into  bone  there  is  the  same  appearance 
of  osteoblasts  in  and  upon  the  tissue  as  described  above,  and 
these  produce  the  bony  substance  in  the  same  way.  The  result 
in  either  case  is  the  same,  and  it  is  not  possible  by  histological 
means  to  distinguish  between  cartilage  bones  and  membrane 
bones  ;  this  depends  entirely  upon  development.  As  will  appear 
later,  the  distinction  between  the  two  is  very  important. 

Increase  in  the  size  of  membranes  and  cartilage  is  accom- 
plished by  additions  to  the  exterior  as  well  as  by  increase  in 
the  interstitial  substance.  In  the  case  of  bone  this  interstitial 


134      MORPHOLGY  OF   THE   ORGANS   OF   VERTEBRATES. 


increase  is  impossible.  Increase  in  size  is  effected  here  by 
additions  to  the  exterior,  and  in  the  case  of  the  long  bones, 
bodies  of  the  vertebrae,  etc.,  by  the  appearance  of  more  than 
one  centre  of  ossification  in  the  cartilage.  From  these  centres 
ossification  extends  in  all  directions,  but  for  a  time  there  remains 
a  cartilaginous  region  between  the  ends  (epiphyses)  and  the 
main  portion  in  which  increase  in  length  is  possible.  Later 
these  epiphyses  usually  become  so  united  or  anchylosed  to  the 
main  portion  that  the  line  of  division  cannot  be  traced. 

The  skeleton  may  be  divided  into  internal  and  dermal  por- 
tions, and  the  internal  in  turn  is  composed  of  an  axial  portion, 
including  the  vertebral  column,  skull,  ribs,  and  breast-bone  ; 
and  an  appendicular  portion,  consisting  of  the  skeleton  of  the 
appendages  and  the  girdles  supporting  them. 

The  vertebral  column  is  developed  around  the  notochord 
(p.  17).  This,  as  will  be  remembered,  is  a  rod-like  structure 
of  entodermal  origin  which  lies  between  the  alimentary  tract 
and  the  central  nervous  system,  extending  from  just  behind  the 
infundibulum  to  the  posterior  end  of  the  body.  Its  cells  gradu- 
ally become  gelatinous,  and  migrate 
toward  the  periphery,  where  they 
finally  become  arranged  in  a  man- 
ner recalling  epithelium  ;  while  the 
mass  of  the  notochord  is  composed 
of  a  reticulum,  in  the  meshes  of 
which  is  the  rather  solid  jelly. 
The  cellular  envelope  thus  formed 
and  its  derivatives  are  frequently 
called  the  elastica  interna.  It  is 
clearly  of  entodermal  origin.  The 
notochord  has  different  fates  in 
the  various  divisions  of  the  verte- 
brates, as  will  be  detailed  later.  In  the  cyclostomes  it  con- 
tinues to  increase  in  size  throughout  life,  and  constitutes  the 
major  portion  of  the  skeletal  axis ;  but  in  other  vertebrates  the 
development  of  vertebrae  relegates  it  to  a  very  subordinate 
position  in  the  adult,  where  it  may  persist  as  a  very  inconspicu- 
ous remnant. 


Fig.  139.  Section  through  noto- 
chord of  embryonic  shark  {Acan- 
thias}.  C,  centrum  of  vertebra; 
E,  /,  elastica  externa  and  interna ; 
Ny  neural  process. 


SKELETON. 


135 


The  vertebrae  proper  arise  from  me_senchymatous  cells,  which 
bud  off  as  sclerotomes  (p.  102  and  Fig.  1 1 1)  from  the  developing 
mesothelial  tissues.  Some  of  these  cells  arrange  themselves 
as  a  continuous  envelope  around  the  notochord  (the  notochordal 
sheath  or  elastica  externa),  while  others  wander  inwards,  be- 
tween the  spinal  cord,  notochord,  and  muscle  plates.  It  is  to 
be  noted  that  this  skeletogenous  tissue  loses  all  segmental  char- 
acter, and  that  the  segmentation  later  to  be  seen  in  the  vertebras 
is  secondary,  and  is  the 
result  of  the  relations  of 
myotomes  and  nerves. 
In  the  cyclostomes  the 
notochordal  sheath  in- 
creases in  thickness 
with  age,  and  in  these 
forms  reaches  its  highest 
development. 

The  earliest  appear- 
ance of  the  segmental 
skeletal  structures  is 
seen  as  an  increasing 
density  of  the  mesen- 
chyme  between  the  in- 
ner surface  of  each  J*G'  I4°'  Section  through  a  developing  ver- 

tebral  centre  of  the  pig,  showing  the  multiplication 

myotome  and  the  Spinal  of  the  mesenchyme  cells  where  cartilages  are  to 
COrd.  These  more  dense  arise.  C,  vertebral  centrum;  Z>,  dorsal;  F,  ven- 

portions    are    soon  con-    tral  roots  of  a  sPinal  nerve  ;  ^  Sanglion  of  dorsal 

root;   Ar,  notochord ;  A',  rib;   RD,RV,  rami  dor- 

verted  into  cartilage,  the    salis  and  ventralis  of  nerve> 
result  being  a  series  of 

pairs  of  backwardly  directed  rods  (the  neural  processes  or  neu- 
rapophyses),  which  tend  to  arch  in  the  spinal  cord.  A  little 
later  similar  condensations  of  mesenchyme  take  place  around 
the  notochord,  a  ring  of  this  tissue  occurring  opposite  to  each 
pair  of  myotomes.  This  forms  the  rudiment  of  the  body  or 
centrum  of  the  vertebra.  Its  subsequent  history  varies  greatly 
in  different  groups  ;  and  the  final  account  cannot  be  written 
until  we  know  more  of  the  development,  especially  in  the 
ichthyopsida.  As  usually  described  these  membranous  rings 


136     'MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 

become  directly  converted  into  cartilage  and,  in  the  higher 
forms,  into  bone;  but  the  little  we  know  of  development,  to- 
gether with  the  conditions  occurring  in  the  ganoids,  and  espe- 
cially in  certain  fossil  amphibia  (stegocephali),  make  it  probable 
that  a  vertebral  body  or  centrum  is  more  complicated  than  it 
was  once  thought  to  be. 

The  most  complicated  condition  known  is  found  in  the  fossil 
Archegosaurus.     Here  there  occurs  on  the  dorsal  surface  of  the 


FIG.  141.  Diagram  of  rhachi- 
tomous  vertebrae,  based  on  Arche- 
gosaurus. /ia,  haemal  process  ;  hct 
hypocentrum  arcale  ;  hp,  hypo- 
centrum  pleurale ;  np^  neural  pro- 
cess ;  ns,  neural  spine  ;  /,  pleuro- 
centrum ;  z,  zygapophysis. 


FIG.  142.  Trunk  vertebra  of 
extinct  stegocephalous  Eurycormus 
speciostiS)  showing  rhachitomous 
condition,  after  Zittell.  h,  hypo- 
centrum  ;  n,  neural  arch  ;  p,  pleuro- 
centrum;  r,  radialia. 


notochord  on  either  side  between  two  successive  neural  processes 
(Fig.  1 41 )  a  skeletal  plate,  —  the  pleurocentrum.  On  the  ventral 
surface,  opposite  the  base  of  the  neural  process,  is  an  arched 
band,  the  hypocentrum  *  (or  hypocentrum  arcale),  which  extends 
across  the  notochord  from  one  side  to  the  other.  Behind  this 
and  opposite  the  pleurocentra  are  a  pair  of  skeletal  plates,  —  the 
hypocentra  pleuralia.  More  usually  (Fig.  142)  the  hypocentra 
pleuralia  are  absent.  These  forms  belong  to  the  rhachitomous 
type  of  vertebrae. 

In  the  embolomerous  type  (Fig.  143)  a  vertebral  body  is  com- 
posed of  two  rings,  one  of  which  is  directly  opposite  the  base  of 


1  The  terms  centrum  and  intercentrum  often  used  for  these  parts  lead  to  unnecessary 
confusion  ;  the  intercentrum  is  in  most  cases  the  hypocentrum  arcale. 


SKELETON. 


the  neural  process,  the  other  between  two  of  these  rings.  In 
development  (Amia)  this  embolomerous  condition  is  derived 
from  the  rhachitomous  type  by  the  fusion  of  hypocentra  pleura- 
lia  with  the  pleurocentra  to  form  one  ring  (centrum,  auct.~),  while 
the  other  (intercentrum)  is  developed  by  a  dorsal  extension  of 
the  hypocentrum  arcale.  In  others  it 
may  be  that  no  hypocentra  pleuralia  occur, 
the  centrum  arising  by  a  ventral  extension 
of  the  pleurocentra. 

In  the  birds  and  mammals  the  vertebra 
arises  at  first  by  what  has  been  called  a  ver- 
tebral bow,  passing  beneath  the  notochordal 
sheath  and  obliquely  upwards  and  back- 
wards to  the  posterior  limits  of  the  somite, 
while  a  little  later  the  centrum  proper  forms 
behind  the  bow.  This  of  course  suggests  a 
comparison  with  the  rhachitomous  vertebra. 

Concerning  the  fates  of  these  parts  in 
the  higher  vertebrates  there  is  a  difference 
of  opinion.  American  students,  as  a  rule, 

regard  the  pleurocentra  as  giving  rise  to  the  body  of  the  verte- 
brae in  the  amniotes,  the  intercentrum  appearing  as  the  chevron 
bones  well  known  in  mammals.  In  amphibia  and  teleosts,  on 
the  other  hand,  the  vertebral  body  is  said  to  arise  from  the 
intercentrum  ;  i.e.,  from  the  hypocentrum  arcale.  Thus  the  ver- 
tebrae cannot  be  regarded  as  exactly  homologous  throughout  the 
vertebrate  phylum.  Many  European  authorities,  on  the  other 
hand,  claim  that  the  centrum  of  the  vertebrates  arises  from  the 
hypocentrum  arcale,  and  that  the  pleurocentra  either  contribute 
or  give  rise  to  the  anterior  zygapophyses  to  be  mentioned  later. 

A  third  type  of  vertebra  is  the  phyllospondylous,  the  rela- 
tions of  which  to  the  foregoing  has  yet  to  be  made  out.  In  this, 
the  vertebral  body  is  composed  of  right  and  left  halves.  This 
type  is  found  in  the  fossil  Branchiosauridae  (stegocephalous 
batrachia). 

To  the  parts  of  the  vertebrae  so  far  described  others  may  be 
added.  In  the  embryo  a  ligament  (interspinous  ligament)  runs 
the  length  of  the  body  just  dorsal  to  the  spinal  cord.  Where 


FIG.  143.  Tail  verte- 
brae of  extinct  stego- 
cephalous Eurycormus 
speciosus  showing  embo- 
lomerous condition,  after 
Zittell.  h,  hypocen- 
trum; /ia,  haemal  arch; 
«,  neural  arch  ;/,  pleuro- 
centrum  (intercentrum).. 


138     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

this  passes  between  the  dorsal  ends  of  the  neural  processes  it 
becomes  converted  into  cartilage,  thus  giving  rise  to  an  addi- 
tional element  (spinous  process  or  neural  spine),  which,  together 


FIG.  144.     Fifth  to  seventh  caudal  vertebrae  of  Perameles  gunni. 
arch ;    n,   neural   processes ;    t,  transverse  processes. 


haemal 


with  the  two  neural  processes,  form  a  neural  arch  enclosing  and 
protecting  the  spinal  cord.  In  the  caudal  region  of  the  ichthy- 
opsida  and  some  higher  forms,  the  vertebra  is  completed  below  by 
a  similar  haemal  arch,  which  encloses  the  caudal  artery  and  vein. 

This  arch  is  composed  of  a  pair  of  hae- 
mal processes  (haemapophyses)  and  a 
haemal  spine.  These  various  parts  of 
the  vertebrae  arise  separately  ;  but  they 
exhibit  in  recent  forms  a  tendency  to 
fuse  together  in  the  adult,  the  fusion 
being  most  complete  in  the  higher 
groups. 

The  vertebrae  are  laid  down  at  an 
early  stage  in  development,  and  their 
number  is  not  subsequently  increased. 
Increase  in  length  of  body  is  therefore 
Caudal  verte-  accomplished  by  longitudinal  growth  of 
the  centra  of  the  vertebrae.  In  the 

additions   ar 


TIG.  145. 
bra  of  alligator,   az,  prezyga- 

pophysis;     c      centrum;     d, 
diapophysis  ;  h,  haemal  arch; 

ns,  neural  spine;  pz,  postzy-  layers,  on  the  circumference  of  the  cen- 
.^apophysis.  trum  first  formed,  each  new  layer  being 

slightly  longer  than  its  predecessor.    As 

a  result  the  centrum  becomes  concave  on  either  end,  —  is  am- 
l>hiccelous.  The  parts  of  the  centrum  first  formed  prevent 
any  farther  increase-  of  the  notochord  in  the  intravertebral 
regions  ;  but  intervertebrally  it  expands,  filling  up  the  cavities 
between  the  successive  vertebrae,  and  thus  assuming  the  appear- 


SKELETON. 


139 


me 

¥ V£=^W ^ 


FIG.  146.  Diagram  of  method  of  growth 
of  amphicoelous  vertebrae,  r,  centrum ;  /,  in- 
tercentral  enlargements  of,  «,  notochord;  1-4, 
successive  layers  of  centra. 


ance  of  a  string  of  beads  (Fig.  146).     In  the  amphibia  we  have 
at  first  the  conditions  just  described,  and  in  the  perennibran- 
chiate    forms   this    amphi- 
coelous   condition    persists     • ^py~~7^V — 7£-V~"5^El  =-"4 

throughout  life.  Higher  ^^fc^^fc 
still,  there  appears  an  in- 
tervertebral growth  of  car- 
tilage (Fig.  147,  A)  which 
produces  a  secondary  series 
of  constrictions  in  the  no- 
tochord. A  later  stage  in 
the  process  is  shown  in  Fig.  147,  B,  where  an  absorption  of  a 
part  of  the  intervertebral  cartilage  is  taking  place  in  such  a  way 
as  to  result  in  the  formation  of  a  cup  at  one  end  of  the  vertebra, 
and  at  the  other  of  a  rounded  extremity  which  fits  the  cup  at 
the  end  of  the  next  vertebra.  The  extreme  of  the  process  is 
shown  in  Fig.  147,  C,  where  the  intervertebral  cartilage  has  been 
cut  completely  in  two,  the  result  being  the  formation  of  a  ball 

and  socket  joint  be- 
c  ^  tween  the  successive 
vertebrae  ;  while  ossi- 
fication has  extended 
so  far  that  almost  the 
entire  centrum  as  well 
as  a  part  of  the  inter- 
vertebral cartilage  has 
been  converted  into 
bone.  When  this  pro- 
cess results  in  a  cen- 
trum rounded  in  front 
and  hollow  behind, 
we  have  an  opistho- 
oelous  vertebra  ;  when 
rounded  behind  and 
hollow  in  front,  it  is  proccelous.  A  statement  of  the  occurrence 
of  these  three  types  of  vertebrae  centra  may  be  given  here. 

Amphicoelous  :    most  fishes,  most    perennibranch  urodeles, 
some  salamanders,  some  stegocephali,  gymnophiona,  many  di- 


FlG.  I47«  Diagrams  of  developing  vertebrae  of 
urodeles,  modified  from  Wiedersheim.  r,  centrum; 
ck,  chorda;  e,  elastica  externa ;  i,  intercentral  carti- 
lage ;  /,  ligament ;  s,  incisure  in  cartilage.  Bone 
lined,  cartilage  dotted. 


I4O     MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 

nosaurs,1  plesiosaurs,  ichthyosaurs,  precretaceous  crocodiles, 
geckos,  rhynchocephalia,  and  the  fossil  birds  Arch&opteryx  and 
Ichthyornis. 

Opisthocoelous  :  Lepidosteus,  most  salamanders,  Pipa,  Dis- 
coglossus  (anura),  most  dinosaurs,  some  vertebrae  in  penguins 
and  auks,  and  the  neck  vertebrae  of  most  ungulates. 

Procoelous :  the  majority  of  anura,  reptiles,  and  birds. 

In  the  majority  of  mammals  the  vertebrae  are  flat  upon  each 
end  of  the  centrum,  —  amphiplatyan. 

In  forms  with  amphicoelous  vertebrae  there  was  no  true 
articulation  of  the  separate  elements  of  the  vertebral  column  ; 
but  with  the  assumption  of  pro-  or  Opisthocoelous  conditions  the 
vertebral  centra  articulate  with  one  another,  and  frequently 


71.  S. 


FIG.  148.  Anterior  and  posterior  faces  of  a  vertebra  of  Python,  from  Huxley. 
ns,  neural  spine  ;  ptz,  postzygapophysis  ;/#,  prezygapophysis ;  tpt  transverse  process; 
za,  zygantrum;  zs,  zygosphene. 

accessory  portions  are  developed  to  lock  the  vertebrae  more 
firmly  together.  Most  common  of  these  are  what  are  known 
as  articular  processes  (zygapophyses).  Of  these  there  are  two 
pairs,  arising  from  the  anterior  and  posterior  surfaces  of  the 
neurapophyses.  The  anterior  or  prezygapophyses  have  their 
flattened  surface  turned  dorsally  so  that  they  can  articulate  with 
the  ventral  surfaces  of  the  posterior  process  (postzygapophyses) 
of  the  vertebra  in  front.  In  the  snakes  and  some  lizards  (igua- 
nidae)  these  are  re-enforced  by  articular  surfaces  developed  from 
the  neural  spine.  On  the  anterior  surface  of  the  base  of  the 
spine  a  wedge-shaped  process  (zygosphene)  projects  forward,  its 

1  In  Camarasaurus  the  first  caudal  is  convex  on  either  end. 


SKELETON.  141 

articular  surfaces  directed  obliquely  outwards  and  downwards. 
This  fits  into  a  corresponding  cavity  (zygantrum)  on  the  poste- 
rior surface  of  the  neural  spine  of  the  vertebra  in  front. 

In  all  forms  above  fishes,  what  are  known  as  transverse 
processes  (pleurapophyses)  occur.  The  homologies  of  these  are 
not  settled.  In  general  terms  there  may  be  said  to  be  two  of 
these  on  either  side,  a  diapophysis  connected  with  the  neural 
process,  and  a  parapophysis  connected  with  the  vertebral  cen- 
trum. One  or  the  other  of  these  may  excel  in  development, 
and  occasionally  either  may  be  rudimentary.  In  addition  the 
names  anapophysis  and  meta- 
pophysis  have  been  given  to 
certain  projections  upon  the 
neural  processes  which  seem 
to  be  without  great  morpho- 
logical significance. 

The  vertebral  column  or 
backbone  is  built  up  of  these 
vertebrae,  and  in  this  struc- 
ture two  or  more  regions  can 
always  be  clearly  distin- 
guished. In  the  fishes  there  „  FlG'  l^'  Anterior  thoracic  vertebra  of 

alligator.       Cy  canal ;    677,  capitular    head 

are  two  of  these  regions, trunk  of  rib.  CT)  centrum;  D,  diapophysis;  />, 

and  Caudal,  the    Caudal    being    parapophysis;   PO,  postzygapophysis ;  PR, 

distinguished  by  the  presence  P«aygaPoPhysis ;     s,    spinous     process; 

7Y/,  tubercular  head  of  rib ;  VC,  vertebrar- 

of  a  complete  haemal  arch  in  terial  canal> 
connection  with  each  verte- 
bra, while  in  the  trunk  the  haemal  processes  diverge  and  be- 
come converted  into  so-called  ribs  (see  below).  In  the  am- 
phibia two  other  vertebral  regions  —  cervical  and  sacral  —  occur. 
The  sacrum  intervenes  between  trunk  and  caudal  vertebrae,  and 
gives  support  to  the  pelvic  arch  by  which  the  hind  limbs  are 
supported.  The  trunk  vertebrae  bear  true  ribs,  while  the  cervi- 
cal vertebra  lacks  ribs  and  transverse  processes,  or  these  are 
present  in  a  rudimentary  condition.  The  line  between  cervical 
and  trunk  vertebrae  is  also  loosely  drawn  by  the  girdle  of  the 
fore  limb.  In  the  sauropsida  (except  in  the  limbless  forms)  the 
same  regions  can  be  traced  as  in  the  amphibia ;  but  it  is  to  be 


142     MORPHOLOGY  OF   THE    ORGANS  OF   VERTEBRATES. 


FIG.  150.     Skeleton 
of  Necturus. 


noticed  that  both  sacral  and  cervical 
regions  are  increased  in  extent,  there 
being  two  or  three  sacral  and  a  much 
larger  number  of  cervical  vertebrae.  In 
the  mammals  these  regions  are  still  fur- 
ther increased  by  a  division  of  the  trunk 
into  a  thoracic  ('  dorsal ')  region,  the 
vertebrae  of  which  bear  ribs,  and  a  lum- 
bar region  in  which  ribs  are  wanting. 

In  certain  regions  there  is  a  strong 
tendency  towards  the  fusion  of  vertebrae. 
Most  frequently  those  of  the  sacrum 
unite  into  a  single  piece,  while  fusions 
in  the  caudal  region  are  numerous,  and 
are  correlated  with  the  partial  or  entire 
disappearance  of  the  tail.  In  modern 
birds  there  results  from  this  a  short  bony 
complex,  the  pygostyle,  while  in  the 
anura  the  caudal  vertebrae  of  the  tadpole 
are  coalesced  into  the  rod-like  urostyle. 
In  other  regions  this  union  is  less  fre- 
quent ;  but  the  fusion  of  the  anterior 
vertebrae  to  form  the  anterior  vertebral 
plate  of  the  skates  and  the  anchylosis  of 
the  cervical  vertebrae  in  the  whales,  and 
the  occasional  fusion  of  some  dorsals  in 
birds,  will  be  recalled. 

The  anterior  two  vertebrae  in  the 
amniotes  call  for  special  notice.  The 
first  of  these,  which  joins  the  skull,  is 
known  as  the  atlas,  the  second  as  the 
axis  or  epistropheus.  The  atlas  bears 
on  its  anterior  face  articular  surfaces 
for  articulation  with  the  skull ;  its  neu- 
ral arch  is  well  developed,  but  the  cen- 
trum is  absent,  there  being  below  but  a 
thin  bony  arch,  regarded  by  some  as  the 
first  intercentrum  (/>.,  hypocentrum 


SKELETON.  143 

arcale).  It  arises  in  development  from  the  ventral  part  of  the 
vertebral  bow  (p.  1 37).  The  axis  is  in  most  respects  a  normal 
vertebra,  but  it  bears,  projecting  "from  the  anterior  face  of  its 
centrum,  a  more  or  less  cylindrical  outgrowth,  the  odontoid  pro- 
cess ;  this  is  morphologically  the  centrum  of  the  atlas,  which 
has  lost  its  connection  with  its  proper  neural  arch,  and  has 
become  secondarily  united  with  the  centrum  of  the  second 
vertebra,  forming  a  pivot  about  which  the  atlas  turns. 

In  crocodiles,  Hatteria,  and  possibly  some  mammals,  a  pair  of 
plates  (reptiles)  or  a  single  plate  occurs  on  the  dorsal  anterior 
portion  of  the  neural  arch  of  the 
atlas.  This  is  the  so-called  pro- 
atlas  ;  but  whether  this  is  the  last 
remnant  of  a  vertebra  which  has 
otherwise  disappeared  from  be- 
tween the  existing  atlas  and  the 
base  of  the  cranium  cannot  yet  be 
definitely  decided.  Nor  is  it  pos- 
sible as  yet  to  say  whether  the 
only  cervical  vertebra  of  the  am-  FlG.  I5I.  Three  anterior  verte- 

phibia  is  homologous  with  either  brae  of  alligator.  «,  atlas;  e,  axis; 
atlas  Or  axis  of  the  amniotes.  «^dont°id  process;  />,  pro-atlas;  r» 

Ribs.  —  The  name  rib  has  been 

applied  to  two  different  structures,1  one  appearing  in  the  gan- 
oids, teleosts,  and  dipnoi,  the  other  in  amphibia  and  amniotes, 
and  apparently  in  selachii  as  far  as  these  latter  have  ribs. 

The  ribs  of  the  fish  are  the  haemal  processes  of  the  trunk 
vertebrae,  which,  in  the  region  of  the  body  cavity,  extend  from 
the  vertebral  centres  towards  the  ventral  surface  between  the 
muscles  and  the  coelomic  walls.  The  transitions  from  these 
ribs  into  the  haemal  arches  can  be  traced  in  any  fish  skeleton. 
In  the  caudal  region  of  the  urodeles  haemal  arches  comparable 
to  those  of  fishes  are  present,  and  besides  these  the  caudal 
vertebrae  also  bear  transverse  processes  which  extend  directly 
outwards  between  the  epi-  and  hypaxial  muscles.  Following; 

1  The  view  of  the  ribs  adopted  here  is  that  which  appears  to  have  the  better  basis. 
Baur  and  others  hold  that  ribs  are  homologous  throughout  the  vertebrates,  but  their  reasons, 
are  not  conclusive. 


144     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


the  vertebrae  forward,  it  is  seen  that  the  transverse  process  of 
the  sacral  vertebra,  considerably  enlarged,  supports  the  pelvic 
arch,  while  in  the  presacral  vertebrae  these  same  transverse 
processes  bear  short  articulated  elements,  —  the  ribs.  It  follows 
from  this  (i)  that  the  amphibian  ribs  are  not  equivalent  to  the 
haemal  processes  in  these  animals,  and  (2)  that  they  are  struc- 
tures different  from  the  ribs  of  fishes.  This  view  is  farther 
substantiated  by  the  conditions  which  ob- 
tain in  the  ganoid  Polypterus,  where  both 
types  of  ribs,  those  of  fishes  and  those 
of  the  higher  vertebrates,  occur  in  the 
same  segment,  the  latter  lying  in  the 
connective  tissue  between  the  epi-  and 
hypaxial  muscular  systems; 

The  ribs  of  the  amniotes  are  clearly 
homologous  with  those  of  the  amphibia. 
They  are  intersegmental  in  position,  and 
arise  by  a  condrification  and  more  or  less 
complete  ossification  of  part  of  the  myo- 
commatous  tissue,  a  mode  of  development 
which  readily  explains  their  frequent  ex- 
tension to  the  ventral  surface. 

In  the  fishes  the  ribs  (sometimes 
lacking,  as  in  some  plectognaths  and  lo- 
phobranchs)  are  usually  slender,  and  are 
frequently  firmly  united  to  the  vertebral 
centra  ;  or,  again,  they  may  be  movably 
articulated  to  short  'basal  stumps.'  In 
many  physostomous  fishes  some  of  the  anterior  ribs  are  modi- 
fied to  give  rise  to  a  chain  of  bones  connecting  the  air-bladder 
with  the  ear.  Besides  these  ribs,  there  frequently  occur  in 
fishes  slender  bones  in  the  fleshy  portions,  the  homologies  of 
which  remain  to  be  ascertained.  Possibly  some  of  them  may 
represent  the  ribs  of  the  higher  forms.  These  epimerals,  epi- 
centrals,  and  epipleurals,  as  they  are  called,  are  stated  to  be 
without  a  cartilage  stage. 

The  ribs  of  the  elasmobranchs  are  small  and  cartilaginous, 
and  are  more  or  less  intimately  united  with  the  vertebral  centra. 


FIG.  152.  Section 
through  tail  of  Amblys- 
toma,  showing  the  two 
types  of  rib.  ^,  epaxial 
muscles;  h,  haemal 
arches ;  hy,  hypaxial  mus- 
cles; n,  notochord;  r, 
true  ribs. 


SKELETON. 


145 


In  their  relationships  to  the  muscles  they  resemble  the  ribs 
of  the  amphibia,  and  are  in  no  way  differentiations  of  haemal 
arches. 

From  the  amphibia  upwards  the  ribs  are  typically  articulated 
with  the  vertebrae  by  two  heads,  a  dorsal  or  tubercular  head 
articulating  with  the  diapophysis,  a  ventral  or  capitular  head 
resting  upon  the  parapophysis.  There  is  thus  formed  a  skeletal 
arch  (vertebrarterial  canal)  between  rib  and  vertebra,  through 
which  passes  a  vertebral  artery  (Fig.  149  VC).  In  the  am- 


FIG.  153.  Anterior  end  of  the  vertebral  column  of  Polypterus,  showing  both 
kinds  of  ribs  from  below,  from  Wiedersheim.  Ps,  parasphenoid ;  R,  true  ribs 
(1-V);  WK,  vertebral  centra;  +  fish  ribs. 

phibia  the  two  heads  are  said  to  arise  separately  and  to  unite 
later.  From  these  typical  conditions  various  modifications  may 
occur.  Thus  either  head  may  disappear,  while  the  parapophysis 
(as  in  many  mammals)  may  be  reduced  to  an  articular  surface. 
Again,  as  in  the  anura,  the  ribs  may  fuse  to  the  diapophysis,  or, 
as  in  the  neck  of  mammals,  to  both  di-  and  parapophysis.  In 
crocodiles  both  tubercular  and  capitular  heads  articulate  with 
the  transverse  process  in  most  of  the  thoracic  ribs. 

In  the  amphibia  the  ribs  are  usually  short,  and  are  confined 


146     MORPHOLOGY  OF  THE    ORGANS   OF   VERTEBRATES. 

to  the  region  near  the  backbone.1  In  some  forms  (Megaloba- 
trachus  and  some  stegocephalans)  the  ribs  are  not  confined  to 
the  trunk  region,  but  from  three  to  eight  pairs  may  occur  in 
the  tail.  It  is  to  be  noted  that  the  pelvis  does  not  articulate 
directly  with  the  transverse  process  of  the  sacral  vertebra,  but 
that  connection  is  effected  by  the  intervention  of  a  sacral  rib, 
distinct  in  many  forms.  In  the  caecilians  ribs  occur  on  every 
vertebra  except  the  first  and  last. 

In  the  amniotes  the  ribs  in  the  trunk  region  acquire  a  much 
greater  development,  and,  like   the  hoops   of  a  barrel,  extend 


FIG.  154.     Pelvis  and  sacrum  of  alligator.     /,  ilium;  R,  sacral  ribs  ; 
5',  5",  sacral  vertebrae. 

around  the  body  cavity.  They  may  be  ossified  throughout  their 
extent,  in  which  case  each  rib  is  usually  divided  into  several 
segments  (crocodile),2  but  usually  a  considerable  portion  re- 
mains cartilaginous.  Ventrally  they  may  terminate  freely,  or 
they  may  connect  with  a  sternum  to  be  described  later.  In  the 
great  majority  of  the  birds,  as  well  as  in  some  reptiles  (croco- 
dilia,  rhynchocephalia),  each  rib  bears  a  backwardly  directed 
uncinate  process,  which  overlaps  the  rib  behind,  thus  giving 
additional  strength  to  the  thoracic  framework. 

1  Ribs  occur  in  the  ventral  region  of  some  stegocephali  (see  p.  147),  and  cartilaginous 
ventral  ribs  have  been  described  in  Necturus  and  Menopoma  (urodeles). 

2  The  median  segment  in  crocodiles  is  not  truly  ossified,  but  is  cartilage  partially 
calcified. 


SKELETON. 


147 


In  the  cervical  region  the  ribs  are  much  shorter.  They 
may  be  freely  articulated  to 
the  vertebrae  (crocodiles,  etc.), 
but  usually  they  are  coalesced 
to  transverse  processes  and 
centra,  the  foramen  for  the 
passage  of  the  vertebral  artery 
remaining  to  show  the  mor- 
phological relations.  Usually 
caudal  ribs  are  poorly  devel- 
oped, but  in  some  reptiles 
they  may  appear  on  almost 
every  caudal  vertebra. 

In  some  stegocephals,  as 
well  as  in  many  reptiles  (Hat- 
teria,  crocodiles,  ichthyosaurs, 
pterosaurs,  etc.),  so-called  ab- 
dominal ribs  occur.  These 
are  chondrifications  or  ossifi- 
cations in  the  ventral  wall  of 
the  abdomen,  usually  behind 
the  true  ribs,  and  external 
to  the  rectus  muscles.  From 
the  fact  that  these  are  not 
homologous  with  the  true  ribs, 
the  name  gastralia  has  been 
given  them.  They  may,  as  in 
crocodiles,  equal  the  segments 
in  number  ;  they  are  twice  as 
many  in  Hatteria,  while  in 
some  stegocephals  there  are 
several  series  of  ossicles  to 
the  somite. 

Sternum.  —  A  sternum  or 
breast  bone  is  absent  in  all 
fishes,  but  occurs  in  the  ma- 
jority of  the  higher  formes  ;  _  Y^'  l^'  Pos!e'ior  vf  ;eb'al  "i™  of 

:         .     .  Testudo  grceca.     CR,  caudal  ribs ;  /,  ilium ; 

but  it  is  as  yet  an  open  ques-  ^  trunk  ribs;  SR,  sacral  ribs. 


148     MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


tion  as  to  how  far  the  sternum  of  the  amphibia  is  homologous 
with  the  similarly  named  structure  in  the  amniotes. 

In  the  amphibia  the  sternum  arises  as  a  pair  of  longitudinal 
cartilaginous  rods  in  the  connective  tissue  on  the  ventral  sur- 
face of  the  body.  These 
rods  soon  unite,  and  form  an 
unpaired  plate  in  the  median 
line  between  the  origin  of 
the  fore  limbs.  In  the  uro- 
deles  the  sternum  remains 
as  a  small  plate  just  behind 
the  ventral  portion  of  the 
shoulder  girdle,  but  in  the 
anura  it  extends  farther  for- 
ward. Its  median  portion  is 
caught  between  the  epicora- 


FiG.  156.     Sternum   and  ventral  portion 
of  the  shoulder  girdle  of  Rana>  after  Wieders- 


coids,   and   is   reduced   to  a 


heim.    d,  clavicle ;  co,  coracoid ;  ec,  epicora-    very  slender  thread;  but  in 


coid ;  g,  glenoid  fossa ;  os,  omosternum ;  s, 
ventral  part  of  scapula ;  .?/,  sternum ;  .r, 
xiphisternum. 


front  of  the  girdle  it  expands 
again  in  a  plate,  the  so-called 
omosternum.  In  the  uro- 
deles  the  sternum  is  cartilaginous  ;  but  in  the  anura  portions  of 
the  omosternum,  as  well  as  of  the  posterior  portion  (termed 
xiphisternum,  a  term  adopted  from  human  anatomy),  become 
ossified.  The  sternum  is  lacking  in  the  footless  amphibia. 

In  the  amniotes  the  sternum  arises  from  the  ventral  ends  of 
the  ribs.  The  distal  ends  of  these  become  separated  from  the 
rest,  and  unite  to  form  a  pair  of  ventral  rods,  which  then  unite 
to  form  the  unpaired  structure,  which  in  many  forms  shows 
evidences  of  its  origin  from  a  series  of  elements,  —  sternebrae. 
The  sternum  is  lacking  in  snakes  and  turtles.  In  the  dinosaur 
Amphiccelias,  it  is  said  to  have  been  paired  in  the  adult,  the  two 
halves  possibly  having  been  united  by  cartilage.  In  the  lizards 
it  is  usually  a  broad  rhomboidal  plate.  In  the  birds  but  few 
(at  most  eight)  ribs  contribute  to  the  sternum,  which  is  a  broad 
plate,  and  in  the  ordinary  birds  bears  a  strong  keel  or  carina 
upon  its  ventral  surface.  In  the  flightless  birds  the  keel  is 
absent,  and  the  presence  or  absence  of  keel  was  formerly  em- 


SKELETON. 


149 


ployed  as  a  means  of  dividing  birds  into  Ratitae  and  Carinatae. 
It  is  interesting  to  find  a  keel  existing  in  the  bats  and  in  the 
fossil  pterodactyls.  In  the  mammals  the  sternum  is  more 
elongate,  and  more  ribs  contribute  to  its  formation  than  in  the 
sauropsida.  It  may  consist  of  as  many  separate  sternebrae  as 
there  are  ribs  connected  with  it,  or  these  may  so  unite  that  but 
three  separate  bones  can  be  recognized,  a  manubrium  in  front, 
a  body  in  the  middle,  and  an  ensiform  process  (xiphisternum) 
behind,  the  latter  extending  behind  the  ribs. 


FIG.  157.  Ster- 
num of  dog,  show- 
ing sternebrae. 


Fig.  158.  Shoulder  girdle  of  Ornithorhynchus . 
C,  clavicle;  CO,  coracoid;  £,  episternum;  EC,  epi- 
coracoid;  S,  scapula;  ST,  sternum;  J?,  ribs. 


Connected  with  the  sternum  in  many  groups  is  a  structure 
to  which  the  name  episternum  has  been  given.  This  first  appears 
in  the  stegocephali,  but  reaches  its  highest  development  in  the 
reptiles.  It  forms  usually  an  unpaired  plate  connected  with 
the  median  ends  of  the  clavicles,  and  in  those  reptiles  where 
it  occurs  it  is  placed  ventrally  to  the  sternum  proper.  It  is 
expanded  in  front,  and  frequently  takes  the  shape  of  a  T,  the 
arms  supporting  the  clavicles,  while  the  shaft  connects  with  or 
may  even  be  fused  with  the  sternum  proper.  No  episternum 


150     MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 

has  been  described  in  the  birds  ;  but  in  mammals  one  frequently 
exists,  but  here  it  is  placed  anterior  to  instead  of  ventral  to  the 
sternum  proper.  Where  best  developed  it  is  T-shaped  ;  and  it 
may  be  movably  articulated  to  the  sternum  as  in  the  mono- 
tremes  (Fig.  I  58),  or  firmly  united  to  it  (marsupials).  In  cer- 
tain rodents  it  becomes  divided  into  three  parts,  while  in  the 
primates  it  is  reduced  to  the  intermediate  cartilages  by  which 
the  clavicles  articulate  with  the  sternum.  The  omosternum  of 
the  anura  was  formerly  regarded  as  an  episternum,  but  it  is 
apparently  truly  sternal  in  nature. 

The  Skull.  —  The  skeleton  of  the  head,  the  skull,  is  a  very 
complicated  structure  ;  and  in  it  two  regions  may  be  recognized, 
—  a  cranium  for  the  protection  of  the  brain  and  sense  organs 
(eyes,  nose,  ears),  and  a  visceral  skeleton  which  forms  the  jaws, 
and  gives  support  to  the  visceral  walls.  In  the  beginning  all 
of  these  parts  are  outlined  in  cartilage  ;  and  in  marsipobranchs 
and  elasmobranchs  they  never  pass  beyond  the  cartilage  stage, 
although,  as  in  some  sharks,  the  outer  portions  of  the  cartilage 
may  be  calcified1  in  the  adult. /'In  the  higher  groups  this  carti- 
lage may  be  partially  or  almost  completely  converted  into  bone  ; 
and  in  all  vertebrates  above  the  elasmobranchs  there  are  added 
to  those  portions  of  the  skull  which  are  of  cartilage  origin 
numerous  other  skeletal  elements  which  are  not  preformed  in 
cartilage,  but  which  arise  as  ossifications  of  membranes.  It 
therefore  becomes  necessary  to  distinguish  in  the  higher  verte- 
brates between  cartilage-bones  and  membrane-bones,  but  these 
distinctions  can  be  made  only  by  tracing  the  development  ; 
there  is  nothing  in  the  fully  developed  bone  which  will  decide 
the  question. 

In  the  development  of  the  cartilaginous  cranium  (chondro- 
cranium)  there  occurs  first  the  formation  of  a  membranous  cap- 
sule, the  primordial  cranium,  which  encloses  the  brain  and  sense 
organs.  In  no  vertebrate  have  the  details  of  this  membranous 
cranium  been  worked  out.  Later  there  is  a  chondrification  of 
this  primordial  cranium  which  proceeds  from  several  distinct  cen- 
tres, which  may  be  spoken  of  as  the  parachordals,  otic  capsules, 
trabeculae,  and  nasal  capsules. 

1  The  distinction  between  calcined  cartilage  and  bone  is  important. 


. 

SKELETON.  151 

As  will  be  recalled,  the  notochord  extends  forward  as  far 
as  the  infundibulum,  and  its'  anterior  end  is  concerned  in  the 
formation  of  the  chondrocranium.  On  either  side  of  this  struc- 
ture there  develops  a  horizontal  cartilaginous  plate,  the  para- 
chordal  cartilage,  which  grows  out  laterally  until  it  unites  with 
a  cartilaginous  box,  the  otic  capsule,  which  forms  around  the 
sac-like  inner  ear  (p.  71).  From  this  union  of  parachordals  and 
otic  capsules,  there  is  formed  a  trough  which  encloses  the  me- 
dulla oblongata  below  and  on  either  side.  Later,  in  the  typical 
conditions,  the  cartilage  gradually  extends  upwards  and  inwards 


FIG.  159.  Early  chondrocranium  of  Amblystoma,  before  the  formation  of  the 
otic  capsules.  ap,  ascending  process  of  quadrate ;  'bqt  body  of  quadrate ;  dp, 
descending  process  of  quadrate;  m,  Meckel's  cartilage;  n,  notochord;  oc,  of, 
foramina  for  oculomotor  and  optic  nerves;  /,  parachordals;  /,  trabecula;  trc,  tra- 
becular  crest.  From  Winslow. 

from  the  dorsal  surface  of  the  otic  capsules  forming  a  plate  — 
the  synotic  tectum  —  which  roofs  in  this  region  of  the  brain 
above.  To  this  region  there  is  added  (amphibia)  a  vertebra  or 
vertebral  complex,  developed  like  those  of  the  vertebral  col- 
umn, which  becomes  finally  united  to  the  parachordals  and  otic 
capsules,  and  closes  in  the  cranium  behind.  Comparative  mor- 
phology would  also  lead  us  to  regard  the  parachordals  as  formed 
of  coalesced  vertebral  centra  ;  but  in  their  history,  so  far  as  made 
out,  they  of  themselves  afford  not  the  slightest  clew  as  to  the 
number  of  elements  fused  together  in  this  region. 

The  trabeculae  cranii  are  a  pair  of  cartilaginous  rods  which 


152     MORPHOLOGY  OF   THE    ORGANS   OF   VERTEBRAl^ES. 

extend  forward  from  the  anterior  end  of  the  parachordals  (or 
of  the  notochord)  on  either  side  of  the  pituitary  body.  In 
front,  at  about  the  anterior  end  of  the  brain,  these  trabeculse 
turn  inwards  towards  each  other  and  fuse  into  a  median  mass 
which,  from  its  future  history,  is  known  as  the  ethmoid  plate. 
Farther  forward  the  trabeculae  separate,  and  turn  outward  in 
front  of  the  developing  olfactory  organs,  the  diverging  horns 
thus  formed  being  known  as  the  cornua  trabeculae.  The  far- 
ther development  of  the  trabecular  region  differs  considerably  in 
different  vertebrates.  In  general  the  trabeculae  rapidly  increase 
in  height  by  the  development  of  a  crest  upon  the  dorsal  surface, 


FlG.  160.  Chondrocranium  of  embryo  trout  {Salmo  fontinalis}.  k,  hyoid  ;  hy, 
foramen  for  hyomandibular  nerve ;  />,  foramen  for  glossopharyngeal  nerve ;  jvy 
foramen  for  branch  of  jugular  vein;  in.  Meckel's  cartilage;  ns,  nasal  septum; 
rs,  foramen  for  ophthalmicus  superficialis ;  sb,  supraorbital  bar ;  J/,  synotic  tectum ; 
/,  trabecula;  tc,  legmen  cranii.  From  Winsiow. 

and  in  the  elagmobranchs  and  some  ganoids  (sturgeon,  etc.)  this 
process  is  continued  until  the  brain  is  completely  roofed  in 
above.  In  the  teleosts,  amphibia,  and  amniotes  no  cartilaginous 
roof  (tegmen  cranii)  is  found  in  this  region  ; J  and  in  lizards, 
birds,  and  certain  teleosts  the  trabeculae  retain  their  condition  of 
simple  rods  closely  applied  to  each  other.  In  most  other  ver- 
tebrates the  trabeculae  gradually  grow  together  beneath  the 
twixt  and  fore  brains,  thus  forming~a  complete  floor.  In  the 

1  The  history  in  the  Dipnoi  is  not  known.  In  nearly  adult  animals  (Protopterus) 
there  exists  a  longitudinal  rod  of  cartilage  in  the  roof  of  the  skull  which  may  be  the 
remains  of  an  earlier  complete  cartilage  roof.  The  same  may  also  be  true  of  an  isolated 
cartilage  plate  in  the  skull  of  Polypterus, 


SKELETON. 


153 


urodeles,  crocodiles,  lizards,  and  many  teleosts  no  such  carti- 
laginous cranial  floor  is  formed,  the  ventral  wall  of  the  skull 
being  formed  by  membrane  bones  to  be  described  later. 

Cartilage  walls  are  also  found  in  the  optic  and  olfactory  or- 
gans.    Since  motion  is  necessary  in  the  eye,  the  optic  capsule 


FlG.  161.  Dorsal  and  lateral  views  of  the  chondrocranium  of  Amphiurna. 
anp,  antorbital  process;  ap,  ascending  process  of  quadrate;  c,  cornu  trabeculae; 
e,  ethmoid  plate ;  ef,  foramen  for  ductus  endolymphaticus ;  /,  jugular  foramen ; 
/,  lamina  cribrosa;  w,  Meckel's  cartilage;  n,  notochord  ;  oc,  foramen  for  oculomotor 
nerve;  ocp,  occipital  process  (vertebra);  of,  foramen  for  optic  nerve;  /,  para- 
chordal ;  pal,  foramen  for  palatine  nerve ;  //,  foramen  for  ductus  perilymphaticus ; 
</,  quadrate  ;  s,  stapes;  s/>,  stapedial  process  of  quadrate;  t,  trabecula;  trc,  trabecular 
crest ;  V,  VII,  VIII,  foramina  for  V,  VII,  and  VIII  nerves. 

(sclerotic,  p.  83)  never  participates  in  the  formation  of  the  cra- 
nium. The  nasal  capsules,  on  the  other  hand,  unite  with  the 
anterior  ends  of  the  trabeculae  and  with  the  cornua.  They  are 
frequently  extensively  fenestrated.  In  the  vertebrate  series  a 
general  law  may  be  observed.  The  more  completely  the  adult 
skull  is  ossified,  the  less  developed  is  the  chondrocranium. 


154     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

The  visceral  skeleton  consists  of  a  series  of  paired  bars, 
always  preformed  in  cartilage,  in  the  walls  of  the  pharynx  and 
the  oral  cavity.  Formerly  these  arches,  which  partially  or  com- 


;BH 


HM 


iv  b    HI 


4  FIG.  162.  Diagram  of  skull  and  visceral  arches  of  an  Elasmobranch.  a,  audi- 
tory capsule  ;  b,  basibranchial  ;  <r,  keratobranchial  ;  e,  epibranchial  ;  g,  gill  cleft  ;  //, 
hyoid;  km,  hyomandibular  ;  /,  labial  cartilages;  m,  mandible  (Meckel's  cartilage); 
<?,  olfactory  capsule;  /,  pharyngobranchial  ;  pq,  pterygoquadrate  ;  r,  rostrum;  s, 
spiracle;  2,  5,  exits  of  second  and  fifth  nerves;  I—V,  branchial  arches. 

pletely  surround  the  alimentary  canal,  were  compared  more  or 
less  closely  with  the  ribs,  but  that  this  homology  cannot  be  held 

is  shown  by  the  fact  that  the  ribs 
develop  from  the  somatic  mesen- 
chyme  (i.e.,  that  outside  the  coelom), 
while  the  visceral  skeleton  arises 
from  the  splanchnic  mesenchyme. 
This  visceral  skeleton  is  seen  in  its 
simplest  condition  in  the  region  of 
the  gill  clefts  (p.  22),  where  there  is 
developed  a  branchial  cartilage,  a 
rod-like  structure,  between  each  two 
successive  gill  slits.  In  their  sim- 
FIG.  163.  Visceral  arches  of  p}est  condition  these  are  simple  rods, 

Scy  Ilium,  after  Gegenbaur.     BH,     ,  „      A,          , 

•basihyal;   Ct  copula  (united  basi-     bllt   USUally  theX  beCOme  br°ken   UP 

branchials);  E,  epihyai;  //,  hypo-  into  a  series  of  elements,  typically 
hyal;  HM,  hyomandibular;  II  Y,  four  jn  number,  movably  articulated 

•.•,  ,         .,  -,     '  •> 

Wlth.  each    °ther'    and    named'    Pr°- 
ceeding     from     above     downwards, 

pharyngobranchial,  epibranchial,  keratobranchial,  and  hypobran- 
chial.  Between  the  two  hypobranchials  of  each  arch  is  devel- 
oped an  unpaired  piece,  the  copula  or  basibranchial,  and  these 


hvoid  ;     K*     keratobranchial;     P. 


SKELETON. 


55 


copulae  become  more  or  less   intimately  connected  with  each 
other,  thus  forming  a  support  for  the  whole  visceral  skeleton. 

The  two  anterior  arches  have  somewhat  different  fates. 
The  second  (counting  from  in  front)  is  called  the  hyoid  arch, 
and  it  lies  between  the  first  true  branchial  cleft  and  the  spiracu- 
lar  cleft  (Eustachian  tube,  p.  73).  In  the  fishes  this  arch  is 
divided  into  two  primary  pieces,  —  a  dorsal  hyomandibular  and  a 
ventral  hyoid  proper.  The  former  loses  more  or  less  completely 


FIG.  164.  Skull  of  cod,  the  outer  membrane  bones  removed,  after  Hertwig. 
A,  angulare;  AR,  articulare  ;  BR,  branchiostegals  ;  DE,  dentary ;  ££,  ectethmoid  ; 
EKT,  ectopterygoid ;  ENT,  entopterygoid ;  EPO,  epiotic;  FR,  frontal;  Si1-3, 
hyoid;  ffM,  hyomandibular;  IH,  interhyal ;  MAt  maxillary;  ME,  mesethmoid; 
MT,  metapterygoid;  JVA,  nasal;  OCB,  basioccipital ;  OCL,  exoccipital  ;  OCS, 
supraoccipital ;  P,  parietal ;  PA ,  palatine ;  PR O,  prootic  ;  PS,  parasphenoid ;  PTO, 
prootic;  Q,  quadrate;  SPO,  sphenotic;  SY,  symplectic. 

its  connection  with  the  hyoid,  and  intervenes  between  the  jaws 
and  the  cranium,  where  it  forms  the  whole  (elasmobranchs)  or 
a  part  (ganoids  and  teleosts)  of  a  suspensor  apparatus  which 
supports  the  jaws.  In  all  forms  higher  than  the  teleosts  this 
hyomandibular  element  has  apparently  disappeared.1  The  hyoid 
proper  may  divide  into  three  parts,  —  the  epihyal,  keratohyal, 
and  hypohyal,  —  while  a  copula  (basihyal),  larger  than  the  basi- 
branchials,  is  usually  developed,  and  not  infrequently  grows  for- 
ward to  form  an  internal  skeleton  for  the  tongue. 

1  The  stapes  of  the  ear  may  possibly  be  derived  from  the  hyomandibular. 


156     MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 

Each  branchial  arch  may  develop  cartilaginous  outgrowths  — 
the  branchial  rays  —  which  serve  as  supports  for  the  gills. 
These  may  also  occur  upon  the  hyomandibular  arch  in  those 
forms  where  a  spiracular  gill  is  developed  ;  but  in  the  teleosts 
the  rays  of.  the  hyoid  portion  of  the  arch  are  modified  into 
slender  bony  rods  —  the  branchiostegal  rays  —  which  support  a 
membrane  closing  in  the  gills  beneath  ;  while  the  rays  of  the 
hyomandibular  are  represented  by  the  opercular  bones  to  be 
described  below. 

The  most  anterior  of  the  visceral  arches,  the  mandibular 
arch,  has  lost  all  connection  with  the  respiratory  region,  and 
has  divided  into  two  portions,  which  are  bent  on  each  other  so 
that  they  meet  at  a  sharp  angle  behind.  The  upper  of  these  is 
the  pterygoquadrate,1  the  lower  is  MeckePs  cartilage.  These  two 
cartilages  of  the  two  sides  form  the  jaws  in  the  elasmobranchs. 
Both  pterygoquadrate  and  Meckel's  cartilage  frequently  have 
accessory  labial  cartilages  developed  in  connection  with  them. 
These  have  been  interpreted  as  degenerate  arches  in  front  of 
the  mandibular  arch. 

The  foregoing  outline  of  the  cartilaginous  skull  applies  to 
the  gnathostome  forms ;  but  before  going  more  into  detail,  a 


FlG.  165.  Cranium  and  branchial  basket  of  Petrotnyzon,  after  W.  K.  Parker. 
£,  branchial  basket ;  E,  otic  capsule ;  G,  gill  slits ;  A7",  nasal  capsule ;  N7\  noto- 
chord. 

word  must  be  said  concerning  the  cyclostomes.  The  chondro- 
cranium  is  formed  of  parachordals,  otic  capsules,  and  trabeculae, 
the  cranial  cavity  being  partially  roofed  in  by  a  narrow  tegmen, 
the  so-called  occipital  arch.  In  front  the  cranium  is  closed  by 
a  cartilaginous  nasal  capsule.  The  branchial  skeleton  consists 

1  The  term  palatoquadrate  sometimes  applied  to  this  is  a  misnomer,  since  the  palatine 
bone  is  a  membrane  bone. 


SKELETON. 


157 


of  a  complicated  cartilaginous  framework,  the  vertical  bars  being 
united  by  horizontal  rods.  There  occur  in  connection  with  the 
cranium  several  cartilaginous  bars,  while  in  front  a  series  of 
plates  extend  to  the  end  of  the  head.  There  is  no  structure 
comparable  beyond  a  doubt  to  a  hyoid ;  while  instead  of  mova- 
bly  articulated  jaws,  the  mouth  is  supported  by  a  cartilaginous 
ring,  and  a  well-developed  cartilaginous  framework  exists  in  the 
tongue,  while  the  filaments  around  the  mouth  {Myxine)  have 
cartilaginous  supports. 

In  the  elasmobranchs  the  skull  is  never  converted  into  bone, 
although  calcareous  deposits  may  be  formed  in  its  wall.  The 
cranium  is  a  closed  cap- 
sule, sometimes  carti- 
laginous throughout, 
sometimes  with  places 
in  its  roof  (fontanelles), 
which  are  not  chondri- 
fied,  but  are  closed  with 
membrane.  Through 
the  walls  are  openings 
for  the  passage  of 
nerves  and  blood-ves- 
sels, but  there  is  no 
trace  of  division  into 
separate  elements. 
The  pterygoquadrate 
in  the  normal  sharks  is  united  to  the  chondrocranium  by  liga- 
ments and  muscles,  and  by  the  hyomandibular  suspensor.  In 
tne  holocephali,  on  the  other  hand,  pterygoquadrate  and  cra- 
nium are  firmly  anchylosed  in  the  adult  (Fig.  166),  although 
free  in  the  young. 

Above  the  elasmobranchs  bones  appear  in  the  skull,  both  as 
ossifications  of  cartilage  and  as  membrane  bones.  The  more 
constant  and  more  important  of  these  are  as  follows  :  — 

The  chondrocranium  gives  rise  to  four  bones  around  the 
large  opening  (foramen  magnum)  through  which  the  brain  is 
connected  with  the  spinal  cord.  These  are,  below,  the  basi- 
occipital ;  on  either  side  an  exoccipital ;  and  above,  part  of  a 


FIG.  166. 


Skull  of  Chinuzra  monstrosa  (drawn 
from  a  dry  specimen). 


158  MORPHOLOGY  OF  THE  ORGANS  OF  VERTEBRATES. 

supraoccipital.  In  the  floor  of  the  cranium  in  front  of  the  basi- 
occipital  is  a  basisphenoid,  and  in  front  of  this  a  presphenoid. 
Still  farther  in  front,  in  the  region  of  the  ethmoid  plate  and  the 
nasal  capsules,  a  mesethmoid.  In  the  trabeculae  are  developed 
two  bones  on  either  side,  an  alisphenoid  in  front  of  the  otic  cap- 
sules, and  an  orbitosphenoid  in  the  neighborhood  of  the  eye. 
The  otic  capsules  each  ossify  into  three  bones,  —  a  prootic  in 
front,  an  epiotic  above,  and  an  opisthotic  behind.  These  bones 
form  the  floor  and  a  part  of  the  lateral  walls  of  the  skull.  Not 


FIG.  167.  Base  of  skull  of  alligator  (Alligator  Indus},  bo,  basioccipital  ;  bs9 
basisphenoid;  eo,  exoccipital ;  et>  opening  of  Eustachian  tube;  fm,  foramen  mag- 
num; pao,  paroccipital ;  pt,  pterygoid;  q,  quadrate;  qj,  quadratojugal ;  so,  supra- 
occipital;  sy,  squamosal;  /r,  transversum. 

all  of  them  are  always  developed,  and  again  two  or  more  may 
fuse  together  or  with  membrane  bones. 

The  pterygoquadrates  of  either  side  develop  into  a  pair  of 
pterygoid  and  a  pair  of  quadrate  bones,  while  Meckel's  carti- 
lage never  ossifies,  or  at  most  gives  rise  to  an  articulare  on 
either  side,  where  the  lower  jaw  articulates  with  the  quadrate. 
The  other  visceral  arches  may  ossify  to  a  greater  or  less  extent, 
but  the  names  of  the  resulting  bones  are  the  same  as  those 
given  the  cartilages. 

In  all  terrestrial  vertebrates  certain  cartilages  or  bones  are 
developed  in  connection  with  the  ear,  and  the  most  diverse 
views  have  been  advanced  regarding  the  homologies  of  these 
ossicula  auditus.  The  following  account  is  based  upon  personal 


SKELETON. 


159 


studies  of  the  development  of  these  ossicles  in  amphibia,  sau- 
ropsida,  and  mammalia. 

In  the  urodeles,  where  these  elements  first  appear,  the  lateral 
wall  of  the  otic  capsule  is  interrupted  by  an  opening,  the  fen- 
estra  ovalis,  in  which  a  plate,  the  stapes,  is  supported  by  mem- 
brane. In  several  urodeles  and  in  all  caecilians  this  stapes  is 
connected  with  the  quadrate  by  means  of  a  stapedial  process 
(see  Fig.  161,  s,  sp).  This  may  be  called  the  urodele  type ;  and 
it  is  to  be  noted  that  here  no  tympanum  (p.  73)  occurs,  the 
first  postoral  visceral  cleft  undergoing  reduction  in  develop- 
ment. 

In  the  anura  and  sauropsida  the  tympanum  is  well  developed ; 
and  this  is  crossed  by  a  rod,  the  columella,  often  differentiated 
into  three  parts,  which 
reaches  from  tympanic 
membrane  to  stapes, 
which  is  situated  as  in  the 
urodeles.  This  columella 
serves  to  conduct  sound 
waves  across  the  tympanic 
cavity  to  the  internal  ear. 
In  development  it  arises 


FIG.  1  68.     Diagram  of  auditory  ossicles  and'. 
related  parts  in  the   sauropsida,  based  on  em- 


mn, mandibular  branch  of  facialis  ;  q,  quadrate  ; 
s,  stapes;  /,  tympanum. 


behind  the  tympanum, 

and  When  fully  developed      bryos  of  Scekporus  undulatus.     c,  columella; 
it    is    bound    to    the     pOS-      ct,  chorda  tympani;/,  facialis;  h,  hyoid:  hn, 

terior  wall  by  membrane.  hy°id  branch  of  facialis;  '«»  Meckel's  Cartilage; 
It  is  therefore  clearly 
postspiracular  in  charac- 
ter, and  its  connection  with  the  ventral  portion  of  the  hyoid 
(Fig.  1 68)  indicates  that  possibly  it  is  to  be  homologized  with 
the  hyomandibular  of  the  pisces.  In  these  groups  the  quadrate 
acts  as  a  suspensor  of  the  lower  jaw,  and  has  only  a  ligamental 
connection  —  no  articulation  —  with  the  stapes  or  columella. 

In  the  mammals  two 1  ossicula  intervene  between  the  tympanic 
membrane  and  the  stapes.  The  more  internal  of  these  is  the 
incus,  the  outer  the  malleus.  As  will  be  remembered,  the  lower 


1  Frequently  a  third  element  is  mentioned,  the  os  orbiculare  or  os  lenticulare,  which 
arises  in  the  ligament  between  incus  and  stapes. 


l6o     MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


jaw  of  the  adult  is  without  a  quadrate  suspensorium.  In  the 
embryo  mammal,  however  (Fig.  169),  Meckel's  cartilage  is  seen 
to  be  connected  to  the  otic  capsule  by  means  of  a  quadrate, 
from  which  a  stapedial  process  extends  backwards  to  articulate 
with  the  outer  end  of  the  stapes,  in  a  manner  which  strikingly 
recalls  the  relations  in  the  urodeles.  The  proximal  end  of 
Meckel's  cartilage  is  expanded,  and,  besides  articulating  with  the 
quadrate,  sends  a  long  process,  the  future  manubrium,  backr 

wards  and  inwards,  be- 
tween the  tympanum  and 
the  external  auditory  me- 
atus,  i.  e.,  into  the  tym- 
panic membrane.  Later, 
with  the  formation  of 
membrane  bone  (dentary) 
around  the  more  distal 
portion  of  Meckel's  car- 
tilage, the  lower  jaw  ac- 
quires a  new  articulation 
with  the  skull,  on  the 
under  surface  of  the  zy- 
gomatic  process,  while  at 
the  same  time  the  prox- 
imal end  of  Meckel's  car- 
tilage becomes  segmented 
off  from  the  rest,  and 
gives  rise  to  the  malleus. 
The  quadrate,  having  no  longer  to  serve  as  a  suspensorium, 
loses  its  connection  with  the  otic  capsule,  and  becomes  the 
incus.  Incus  and  malleus  extend  into  the  tympanic  cavity 
from  in  front,  i.e.,  are  prespiraeular,  and  cannot  be  homologous 
with  the  anuran  and  sauropsidan  columella.  Further,  it  will 
be  noticed  that  the  ossicula  of  the  mammal  are  on  the  oppo- 
site side  of  the  chorda  tympani  from  what  is  found  in  the  rep- 
tilia  (Fig.  168).  It  is  an  interesting  fact,  the  bearings  of  which 
will  be  alluded  to  later,  that  the  quadrate,  in  both  urodeles  and 
mammals,  retains  its  articulation  with  the  stapes  throughout  life. 
In  the  reptiles  nothing  of  the  sort  occurs. 


PIG.  169.  Diagram  of  auditory  ossicles  and 
xelated  parts  in  the  mammalia,  based  on  the 
embryo  rat.  a,  external  auditory  meatus;  ct, 
chorda  tympani;  f,  facial  nerve;  /£,  hyoid;  hm, 
hyomandibular  nerve  ;  hn,  hyoid  branch  of  faci- 
alis;  »/,  mallear  portion  of  Meckel's  cartilage,  its 
process  extending  down  between  tympanum  and 
meatus;  mn,  mandibular  branch  of  facialis; 
q  (V),  quadrate,  later  incus;  s,  stapes;  sm,  sta- 
pedial muscle ;  /,  tympanum. 


SKELETON.  •     l6l 

In  the  teleostomous  fishes  an  operculum  or  fold  covering  the 
gill  slits  occurs  ;  and  this  is  supported  by  opercular  bones,  which 
in  their  full  development  may  number  four  on  either  side,  —  oper- 
culum, preoperculum,  inter  operculum,  and  suboperculum.  These 
are  cartilaginous  in  origin,  and  are  usually  regarded  as  extremely 
modified  branchiostegals  of  the  hyomandibular. 

The  membrane  bones  which  complete  the  lateral  walls  and 
roof  in  the  cranium  are  :  the  dorsal  part  of  the  supraoccipital 
(when  distinct  called  paroccipital),  and,  proceeding  forwards, 
a  pair  each  of  parietals,  frontals,  and  nasals,  meeting  in  the 


^ -=- • —  ,     — — = — 

;.  170.  Hyoid  and  opercular  apparatus  of  cod.  £,  branchiostegals;  E,  os 
sum;  ff,  hyoid ;  //J/,  hyomandibular;  /,  interoperculum;  O,  operculum; 
'oerculum :  5.  subooerculum. 


FIG 

entoglossum 
P,  preoperculum ;    S,  suboperculum 

middle  line  above,  the  skull  being  terminated  by  a  pair  of  pre- 
maxillaries,  which  also  appear  on  the  ventral  surface.  Lodged 
in  the  angle  between  nasal  and  frontal  is  a  prefrontal  on  either 
side,  while  a  pair  of  postfrontals  are  placed  in  a  similar  position 
between  the  frontals  and  parietals.  Pre-  and  postfrontals  may 
make  up  the  superior  or  inner  margin  of  the  orbit,  or  a  supra- 
orbital  may  intervene  between  them.  Below  the  postfrontal 
and  behind  the  orbit  there  may  be  a  postorbital  which  may 
extend  beneath  the  orbit,  or  the  posterior  margin  of  the  orbit 
may  be  formed  by  a  squamosal  (temporal),  which  extends  up- 
wards in  front  of  the  otic  region  to  reach  the  supraoccipital  and 
parietal.  A  lachrymal  bone  is  more  constant  than  some  that 


1 62     MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 

have  been  named  ;  it  usually  enters  into  the  composition  of  the 
anterior  wall  of  the  orbit,  but  it  may  be  forced  forward  by 
the  prefrontal. 


FlG.  171.  Ventral  and  side  views  of  the  skull  of  Hatteria  (Sphenodon*),  after 
Giinther.  bo,  basioccipital  ;  bs,  basisphenoid  ;  eo,  exoccipital ;  fr,  frontal ;  j,  jugal ; 
/,  lachrymal ;  mx,  maxillare  ;  n,  nasal ;  oo,  opisthotic  ;  pa,  palatine  ;  pf,  prefrontal ; 
pni,  premaxillary ;  po,  postorbital ;  pof,  postfrontal  ;  pt,  pterygoid ;  q,  quadrate;  qj, 
quadratojugal ;  sq,  squamosal ;  z>,  vomer. 

In  the  elasmobranchs  the  upper  jaw  is  formed  by  the  ptery- 
goquadrate  cartilage,  but  in  all  higher  forms  other  elements 
usurp  these  functions.  In  front  there  are  a  pair  of  premaxilla- 
ries  already  mentioned,  and  behind  these  a  pair  of  maxillaries 


SKELETON. 


163 


usually  occur.  These  may  extend  back  to  the  angle  of  the  jaw, 
or  a  jugal  (malar)  and  a  quadratojugal  may  intervene,  the  lat- 
ter connecting  with  the  quadrate,  and  in  some  cases  arising  in 
part  from  an  ossification  of  a  process  of  the  quadrate  cartilage. 
In  the  roof  of  the  mouth  in  front  are  usually  a  pair  of  vomers, 
and  behind  these,  and  extending  back  usually  to  meet  the  ptery- 
goids,  are  a  pair  of  palatines ;  while  in  some  groups  an  os  trans- 


J'a- 


TZT 


0/10 


FiG.  172.  Skull  of  Cyclodus  from  the  side  and  split  through  the  middle,  from 
Huxley.  Ar,  articulare;  BO,  basioccipital ;  BS,  basisphenoid ;  Co,  columella;  D, 
dentary;  £O,  exoccipital ;  EpO,  epiotic ;  Fr,  frontal;  Ju,  jugal ;  MX,  maxillary  ; 
Na,  nasal;  OpO,  opisthotic;  Pa,  parietal;  Pf,  postfrontal ;  PI,  palatine;  Pmx,  pre- 
maxillary  ;  Prf,  prefrontal ;  PrO,  prootic  ;  Pf,  pterygoid  ;  Qu,  quadrate  ;  SO,  supra- 
occipital  ;  Sq,  squamosal ;  Vo,  vomer ;  V,  VII,  exits  of  fifth  and  seventh  nerves. 

versum  occurs,  connecting  the  hinder  portion  of  the  mandible 
with  the  pterygoid.  In  the  ichthyopsida  the  floor  of  the  chon- 
drocranium  does  not  ossify  ;  and  here  the  remainder  of  the  roof 
of  the  mouth  is  formed  by  an  unpaired  membrane  bone,  —  the 
parasphenoid. 

In  the  ganoids  and  all  higher  forms  membrane  bones  form 
around  Meckel's  cartilage,  and  these  form  the  functional  lower 
jaw.  In  their  greatest  development  there  may  be  several  of 


164     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


these  bones  on  either  side,  —  a  dentary  in  front,  a  splenial  far- 
ther back  on  the  inner  side,  and  an  angulare  extending  forward 
from  the  angle  of  the  jaw  to  meet  the  other  two.  In  addition, 
a  supraangulare  is  sometimes  present  behind  the  articulation  of 
the  lower  jaw  with  the  quadrate. 

Several  of  these  membrane  bones  may  bear  teeth.  When 
teeth  are  present  they  almost  universally  occur  on  the  premaxil- 
laries,  maxillaries,  and  dentary ;  but  they  may  also  occur  on  the 
vomers,  palatines,  parasphenoid,  and  splenials,  and  occasionally 
on  the  pterygoids. 

This  leads  to  the  question  of  the  phylogenetic  origin  of  these 
membrane  bones  of  the  skull.  All  the  evidence  goes  to  show 

that  not  only  these  teeth- 
bearing  bones,  but  most  of 
the  covering  bones  of  the 
skull,  have  arisen  from  the 
fusion  of  dermal  plates, 
much  like  the  placoid  scales 
of  the  elasmobranchs.  In 
the  jaws  the  enamel-capped 
spines  have  given  rise  to 


FIG.  173.    Development  of  dermal  (maxil- 
lary) bone  in  Amblystoma  by  fusion  of  the 


the    teeth,  while   the    basal 


bases  of  teeth.      bt  bone;    c,   cartilage;    d,     plates,  fusing  together,  form 
dentine  of  tooth ;  e,  epidermis ;  /,  tooth.  the     bones    themselves.       In 

many  forms   this   origin    of 

the  bones  by  the  fusion  of  the  bases  of  the  teeth  can  readily  be 
seen  (Fig.  173).  In  the  covering  bones  of  the  cranium  the 
dental  portion  has  disappeared.  The  remaining  membrane 
bones  have  arisen  around  the  canals  of  the  lateral  line  system, 
the  suborbital  chain  of  bones  being  the  most  constant  of  these. 
Through  the  walls  of  the  skull  formed  by  these  cartilage 
and  membrane  bones  are  foramina  for  the  passage  of  nerves  ; 
and  these  openings  afford  important  landmarks  for  the  identi- 
fication of  certain  bones,  especially  in  those  numerous  cases 
where  different  elements  fuse  together.  The  optic  nerve  passes 
through  the  orbitosphenoid.  Between  the  orbitosphenoid  and 
alisphenoid  is  an  opening  (sphenoidal  fissure  or  foramen  lacerus 
anterior)  through  which  pass  the  third,  fourth,  and  sixth,  and 


SKELETON. 


I65 


the  ophthalmic  branch  of  the  fifth  nerves.     The  maxillaris  su- 
perior and  mandibularis  branches  of  the  fifth  nerve  leave  the 


FIG.  174.  Developing  bones  in  the  head  of  Arnphiuma.  Cartilage  dotted, 
bone  lined.  ao,  antorbital  process ;  an,  angulare ;  d,  dentary ;  f,  frontal ;  fa,  fora- 
men ovale ;  mx,  maxillary ;  o,  occipital  vertebra  ;  oc,  otic  capsule ;  pa,  parietal ; 
pm,  premaxilla;  q,  quadrate;  sq,  squamosal ;  st,  stapes;  /,  trabecula;  //,  etc.,  exits 
of  nerves. 

skull  through  the  alisphenoid  bone,  either  through  a  common 
opening  or  through  two  separate  foramina  (f.  ovale  for  the 
mandibular,  f.  rotundum  for  the  other).  The  seventh  nerve 


FIG.  175.  Diagram  of  the  relations  of  the  bones  in  the  mammalian  skull, 
after  Flower.  AS,  alisphenoid;  BH,  basihyal ;  BO,  basioccipital ;  BS,  basi- 
sphenoid;  CH,  ceratohyal ;  EH,  epihyal  ;  EO,  exoccipital ;  F,  frontal;  ^,  jugal ; 
L,  lachrymal ;  MD,  mandible;  MC,  Meckel's  cartilage;  ME,  mesethmoid;  MX, 
maxilla;  N,  nasal;  OS,  orbitosphenoid ;  PA,  parietal;  PL,  palatine;  PM,  pre- 
maxilla ;  PS,  presphenoid ;  FT,  pterygoid ;  S,  squamosal ;  SH,  stylohyoid ;  SO, 
supraoccipital;  T,  turbinal ;  TH,  tympanohyal ;  THH,  thyrohyal;  V,  vomer  ; 
I- 1 2,  exits  of  the  cranial  nerves. 


1 66     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


passes  through  the  otic  bones  (petrosal),  the  ninth  and  tenth 
through  the  jugular  foramen  formed  by  the  junction  of  basi-  and 
exoccipital  and  otic  bones.  Occasional  variations  from  these 

conditions  occur  ;  for  instance,  the  optic 
nerve  may  pass  through  a  notch  in  the 
orbitosphenoid,  or,  again,  the  ophthal- 
mic branch  of  the  fifth  may  be  enclosed 
in  the  alisphenoid. 

While  the  question  of  the  segments 
of  the  head  will  be  taken  up  in  a  later 
section  of  this  volume,  it  may  be  well 
to  point  out  here  that  the  bones  of  the 
skull  form  a  series  of  rings  surrounding 
the  brain  ;  but  it  is  to  be  noticed  that 
these  rings  are  formed  in  part  of  mem- 
brane bones,  in  part  of  cartilage  bones. 
The  posterior  of  these  rings  is  formed 
of  basi-,  ex-,  and  supraoccipitals  ;  next 
in  front  comes  a  ring  formed  of  the 
basisphenoid,  alisphenoids,  and  parie- 
tals-;  third,  one  of  presphenoid,  orbito- 
sphenoids,  and  frontals  ;  and  lastly,  one 
of  ethmoid  and  nasals. 

In  the  fishes,  stegocephalans,  and 
gymnophiona  the  membrane  bones 
form  a  continuous  layer  on  the  outside 
of  the  skull ;  but  in  the  higher  verte- 
brates gaps  may  occur  here  and  there 
behind  the  orbit,  the  fossae  thus  formed 
being  bounded  by  arches  of  bone. 
There  may  be  two  of  these  fossae,  — 
a  more  dorsal  supratemporal,  and  a 
more  ventral  and  lateral  infratemporal. 
These  fossae  are  separated  from  the 

orbit  by  a  bridge  of  bone,  usually  consisting  of  postorbital  and  a 
process  of  the  jugal.  The  infratemporal  is  bounded  externally 
by  a  zygomatic  arch  into  which  the  quadrat ojugal,  jugal,  and 
maxillary  may  enter ;  while  between  the  two  fossae  is  an  arch 


FIG.  176.  Skull  of  the 
Dinosaur  Hadrosaurus,  show- 
ing supra-  (S T~)  and  infra- 
temporal  fossae  (/71),  after 
Cope.  FR,  frontal  ;  y,  jugal, 
N,  nasal ;  O,  orbit ;  P,  post- 
orbital ;  -PA,  parietal;  PM, 
premaxilla;  POF,  post- 
frontal  ;  PJRF,  prefrontal ;  Q, 
quadrate  ;  5,  squamosal ;  SO, 
supraoccipital. 


SKELETON. 


I67 


usually  composed  of  squamosal  and  postorbital.  By  alteration 
in  the  position  or  extent  of  the  bones  these  two  fossae  may  unite 
into  a  single  temporal  fossa,  and  again,  the  boundaries  between 
this  and  the  orbit  may  become  broken  through,  the  postorbital 
arch  being  imperfect  or  totally  disappearing.  (For  details  see 
Reptilia.) 

Appendicular  Skeleton.  —  The  appendages  of  the  vertebrates 
(fins  or  limbs)  arise  as  paired  outgrowths  from  the  sides  of  the 
body,  one  pair,  the  anterior  or  pectoral,  arising  a  short  distance 
behind  the  pharyngeal  region,  the  other,  or  pelvic  (ventral)  pair, 


FIG.  177.     Developing  fin  of  trout,  after  Corning,    f,  fin;    ;;/,  myotomes;  n, 
notochord  ;  the  myotomes  are  seen  to  be  proliferating  strands  of  cells  into  the-fin. 

a  little  in  front  of  the  vent.  In  the  higher  vertebrates  each 
limb  grows  out  as  a  simple  bud,  but  in  some  elasmobranchs 
the  appendages  arise  as  differentiations  of  a  continuous  lateral 
fold  on  either  side  of  the  body.  Into  these  outgrowths  migrate 
cells  derived  from  the  muscle  plates  (Fig.  177),  which  are  to 


1 68     MORPHOLOGY  OF  THE    ORGANS  OF   VERTEBRATES. 


give  rise  to  the  muscles  of  the  appendage,  and  also  mesenchy- 
matous  tissue,  which  becomes  transformed  in  part  into  the 
skeleton.  This  skeleton  is,  with  the  exception  of  the  clavicles, 
preformed  in  cartilage,  the  cartilage  formation  beginning  at 
about  the  middle  of  the  limb  and  proceed- 
ing thence  in  both  directions. 

The  skeletons  of  both  pectoral  and  pelvic 
appendages  are  closely  similar  in  structure. 
Each  consists  of  a  skeletal  arch  or  girdle 
within  the  trunk,  each  girdle  supporting  the 
skeleton  of  the  appendage.  These  girdles 
are  known  respectively  as  the  pectoral  (shoul- 
der) and  pelvic  girdles. 

The  pectoral  girdle  occurs  in  its  simplest 
form  in  the  lower  fishes,  where  it 
is  a  U-shaped  arch  of  cartilage,  the 
bottom  of  the  U  crossing  the  ven- 
tral surface  of  the  body  beneath 
the  skin,  the  arms  projecting  up- 
wards on  either  side,  and  the  ends 
being  connected  by  muscles  with 
the  vertebral  column.1  The  skele- 
ton of  the  fin  is  articulated  to 
either  half  of  the  girdle,  the  point 
of  articulation  being  usually  exca- 
vate, and  known  as  the  glenoid 
fossa.  This  fossa  serves  to  divide 

FIG.  178.     Shoulder  girdle  and       each   half    °f    the  girdle  int°  a  dor- 

proximai  part  of  pectoral  fin  of     sal    or   scapular  and  a  ventral    or 

skate    (Kaia).      g,    right    half    of       coracoid  portion. 

With  the  appearance  of  bone 
(ganoids,  teleosts)  each  half  of 
the  girdle  develops  two  cartilage 

bones,  —  a  scapula,  and  a  second,  usually  regarded  as  a  coracoid; 
while  the  two  halves  of  the  girdle  proper  become  separated  from 
each  other.  In  the  dipnoi,  ganoids,  and  teleosts,  these  are  re- 

1  In  the  skates  the  pectoral  girdle  becomes  attached  to  the  backbone  by  means  of  a 
so-called  suprascapula.  In  many  other  fishes  it  is  connected  with  the  skull  by  a  chain  of 
bones.  Elsewhere,  except  in  some  fossil  reptiles,  it  is  free  from  the  axial  skeleton. 


girdle ;  ms,  mesopterygium  ;  ;///, 
metapterygium;/,  protopterygium; 
r,  radii  of  fin. 


SKELETON. 


169 


enforced  by  membrane  bones.     The  chief  and  largest  of  these 

is  the  cleithrum  (usually  called  the  clavicle),  developed  on  the 

outer  anterior  surface  of  the  girdle,  the  cleithra  of 

the  two  sides  frequently  uniting  below.     To  this  is 

added    above    a    supraclavicle,  which    may   connect 

directly,  or  by  the  intervention   of    a  posttemporal 

bone,  with  the  base  of  the  skull.     Other  membrane 

bones  —  postclavicle,  infraclavicle,  etc.  —  sometimes 

occur. 

In  the  amphibia  and 
higher  groups  other  portions 
may  be  differentiated  in  the 
pectoral  girdle,  and  as  yet 
these  cannot  all  be  homolo- 
gized  with  the  conditions 
found  in  fishes.  In  fact,  it 
is  probable  that  no  detailed 
homology  exists.  The  scapu- 
lar portion  of  the  arch  may 
ossify  throughout,  or  the  os- 
sification may  be  restricted  to 

that  portion  —  the  scapula  —  nearest  the  glenoid  fossa,  while 
the  dorsal  portion  may  be  a  distinct  element,  partly  or  entirely 
cartilaginous,  —  the  suprascapula.  The  ventral  portion  of  the 
girdle  gives  rise  typically  to  two  elements,  a  posterior  coracoid 


FIG.  179.  Shoulder  girdle  of  carp 
(Cyprinus  carpio},  after  Gegenbaur. 
CL>  cleithrum  ;  E,  scapulare  ;  PC,  cora- 
coid (procoracoid)  ;  F,  foramen  between 
coracoid  and  cleithrum ;  A,  attachment 
of  fin. 


FIG.  180.  Pectoral  girdles  of,  A, 
Archegosaurus  and,  B,  Paltfohatteria.  c, 
coracoid  ;  d,  clavicle  ;  e,  episternum  ; 
s,  scapula,  after  Credncr. 


FIG.  181.  Shoulder  girdle,  etc., 
of  Bombinator  igneus,  after  Wied- 
ersheim.  c,  clavicle;  to,  coracoid; 
ec,  epicoracoid ;  g;  glenoid  fossa ; 
pc,  procoracoid;  .?,  scapula;  w, 
suprascapula ;  s/,  sternum. 


MORPHOLOGY  OF   TJfE   ORGANS   OP    VERTEBRATES. 


and  an  anterior  procoracoid,  both  extending  inwards  ;  and  fre- 
quently the  inner  ends  of  these  are  united  by  a  longitudinal 
cartilaginous  band,  —  the  epicoracoid.  To  these  may  be  added 
a  clavicle,  developed  from  membrane,  in  front  of  the  proco- 
racoid, extending  in- 
wards from  the  scap- 
ula, and  usually  con- 
necting  with  the 
sternum  by  means  of 
the  episternum. 
These  parts  undergo 
various  modifications, 
and  some  or  all  of 
them,  with  the  excep- 
tion of  the  scapula, 
may,  here  and  there, 
more  or  less  com- 
pl  e  t  ely  disappear. 
Possibly  the  most 
common  is  the  re- 
placement of  the 

FlG.  182.     Shoulder  girdle   of    Ornithorhynchus. 

C,    clavicle;    CO,    coracoid;    E,    episternum;    EC,       procoracoid      by      the 
epicoracoid;  S,  scapula;  STt  sternum;  ^,  ribs.  clavicle.       The  details 

of  these  modifications 

will  be  given  in  connection  with  the  groups  in  which  they 
occur ;  but  in  the  majority  the  two  halves  of  the  pectoral 
girdle  are  more  or  less  firmly  united  by  means  of  the  sternum. 
The  pelvic  girdle  presents  many  similarities  to  the  anterior 
.arch.  In  the  elasmobranchs  there  is  the  same  transverse  arch 
as  in  the  shoulder  girdle  ;  and  this  supports  the  ventral  fins, 
there  being  in  some  cases  a  dorsal  portion  extending  beyond 
the  fossa  (acetabulum)  in  which  the  fin  articulates.  There  thus 
arise  a  dorsal  iliac  portion  and  a  ventral  ischio-pubic  portion  in 
«ach  half  of  the  arch,  the  ventral  part  being  perforated  by  an 
opening  (obturator  foramen)  for  the  obturator  nerve.  In  the 
other  fishes  the  pelvic  girdle  is  much  less  developed,  and  in  the 
teleosts  the  fins  are  supported  by  these  enlarged  basal  elements 
(vide  infra).  In  the  amphibia  and  higher  groups  the  iliac  por- 


SKELETON. 


171 


tion  is  well  developed  ;  and  when  bones  are  developed  in  the  car- 
tilage, three  elements  can  be  recognized  in  each  half,  —  a  dorsal 
ilium,  and,  below,  an  anterior  pubis  and  a  posterior  ischium,  the 
obturator  foramen  either  forming  a  part  of  the  opening  between 
these  two  bones,  or  passing  through  the  pubis  itself.  Ventrally 
these  bones  can  unite  with  their  fellows  of  the  opposite  side  in 
a  symphysis,  while  all  three  of  a  side  meet  in  the  acetabulum.1 
These  parts  can  be  well  compared  with  those  of  the  pectoral 
girdle  ;  pubis  with  procoracoid,  ischium  with  coracoid,  and  ilium 
with  scapula  ;  but  one  difference 
is  to  be  noted,  —  the  ilium  be- 
comes connected  with  the  sacral 
vertebra  or  vertebrae  by  the  inter- 
vention of  short  ribs  (p.  1 46).  To 
these  parts  in  the  amphibia  there 
is  frequently  added  in  front  of 
the  pubis  a  cartilaginous  epipubis. 
This  reappears  again  in  certain 
reptiles  ;  and  in  mammals  it  may 
be  homologous  with  the  so-called  marsupial  bones,  which  project 
forward  from  the  anterior  margin  of  the  pubis,  there  being  two 
views  upon  this  point.  These  parts  may  undergo  many  modi- 


FIG.  183.  Side  view  of  pelvis  of 
opossum,  after  Minot.  Ac,  acetabu- 
lum ;  y,  obturator  foramen  ;  /Z,  ilium ; 
/S,  ischium  ;  M,  marsupial  bone. 


FIG.  184.     Modifications  of  branchial  arch  and  rays  according  to  Gegenbaur's 
archipterygium  theory. 

fications,  but  the  pelvis  is  not  re-enforced  by  membrane  bones 
such  as  play  such  a  part  in  the  shoulder  girdle. 

1  In  many  mammals  a  distinct  acetabular  bone  occurs  at  the  junction  of  the  three. 


1/2     MORPHOLOGY  OF  THE   OX  CANS   OF   VERTEBRATES. 


FIG.  185.     Skeleton  of  pectoral  fin  of  Ceratodus,  after  Gunther. 

Before  beginning  the  account  of  the  skeleton  of  the  limbs 
it  may  be  well  to  summarize  the  two  prominent  theories  of  the 
origin  of  these  parts.     According  to  the  view  of 
Gegenbaur  (which  he  has  lately  repeated),  limbs 
have  arisen  from   gill  structures   which  have  mi- 
grated backwards.      The  gill  arches    have  given 
rise  to  the  girdles,  while  the  skeletal  parts  of  the 
appendages    have  had  their  origin   from  the  gill 
rays.      With  the  outgrowth  of  the  limb  one  of  the 
gill  rays  near  the  middle  of  the  arch  has  cor- 
respondingly elongated,   and    in  its   outgrowth 
has  carried  the  neighboring  gill  rays  along  with 
it,  the  result  being  a  skeletal  axis  to  the  limb, 
on  either  side  of  which  was  a  series  of  smaller 
skeletal    pieces    (Fig.    184).     A    fin    closely 
corresponding  to  the  requisites  of  this  view 
is  found  in  the  dipnoan  Ceratodus.     By  sup- 
pression   of    almost    all    of    these  accessory 
skeletal  parts  on  one  side  of  the  axis,  and 
a    modification    or    suppression    of    some 
on  the  other  side  of  this  archipterygium, 
Gegenbaur  derives  all  types  of  vertebrate 
limbs.     Fig.    186  shows    the   relations   of 
the  typical  pentadactyle  leg. 

The    other    view,    first    advanced    by 
Thacher,  assumes  that  the  ancestral  verte- 
brate was  provided  with  two  longitudinal 
folds    on    either    side    of  the   body.     The 
more    dorsal    of    these    migrated    upwards, 
those  of  the  two  sides  uniting  to  form  the  dorsal  part  of  the 


FIG.  186.  Diagram 
of  amphibian  fore  limb, 
after  Gegenbaur.  c, 
centrale  ;  //,  humerus  ; 
R,  radius ;  t,  interme- 
dium ;  r,  radiale ;  U, 
ulna;  «,  ulnare ;  1-5, 
carpales.  The  heavy 
line  is  the  axis  of  Gegen- 
baur's  archipterygium, 
the  dotted  lines,  of  the 
radials  of  his  scheme. 


SKELETON-. 


173 


median  fin  to  be  described  later.  From  the  more  ventral  folds 
arose  the  ventral  portion  of  the  median  fin,  behind  the  vent ; 
while  the  pectoral  and  ventral  fins  arose  as  differentiations 
from  the  preanal  region  of  the  folds.  In  fact,  several  existing 
elasmobranchs  exhibit  exactly  this  condition  in  their  develop- 
ment. 

In  those  parts  of  the  fold  where  the  fins  are  to  form,  rod- 
like  cartilage  supports  arose,  possibly  agreeing  in  number  with 
the  myotomes  concerned  in  the  formation  of  the  appendage. 
At  first  these  were  separate  and  nearly  equal  in  size,  but  later 


FlG.  187.  Diagram  of  the  origin  of  median  and  paired  fins,  from  Wiedersheim. 
A,  with  continuous  fin  folds;  B,  with  differentiated  fins.  AF,  anal  fin;  An,  anus; 
BF,  ventral  fin  ;  BrF,  pectoral  fin  ;  D,  dorsal  folds  ;  FF,  adipose  dorsal  ;  RFt 
dorsal  fin  ;  S,  lateral  folds  ;  SF,  caudal  fin. 

the  basal  portions  became  larger  and  separated  from  a  more 
distal  (radial)  part.  Such  a  condition  is  seen  in  the  extinct  elas- 
mobranch  Cladoselache  (Fig.  188)  ;  but  usually  the  basalia  fuse 
into  a  few  larger  elements,  connecting  the  radialia  together,  and 
giving  stiffness  to  the  whole  fin.  One  of  these  enlarged  basalia 
acquired  prominence  over  its  fellows,  and  growing  in  toward  the 
median  line  fused  with  a  similar  ingrowth  from  the  opposite  side, 
thus  giving  rise  to  the  ventral  portion  of  the  girdle.  But  such 
a  bar  would  prove  too  rigid,  and  would  prevent  the  fin  from 
moving  freely,  so  there  appeared  a  joint  on  either  side,  the  distal 
portion  now  articulating  with  the  median  or  girdle  region  at  a 
place  known  as  the  glenoid  fossa  or  acetabulum.  The  skeleton 


1/4      MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 


of  the  fin  proper  would  now  consist  of  the  basals  and  radials 
plus  a  number  of  delicate  rods  of  dermal  origin,  —  the  so-called 
actinotrichia.  From  such  an  archetypal  structure  the  paired 
fins  of  all  fish-like  forms  can  readily  be  deduced  ;  but  the  transi- 
tion between  this  and  the  limb  of  the  higher  vertebrates  offers 


Vi 

FIG.  188.  A,  pectoral,  and  B,  ventral  fins 
of  Cladoselache,  after  Dean,  b,  basalia ;  r, 
radialia. 


many  difficulties,  and  the  interme- 
diate forms  have  not  yet  been 
found.  The  most  plausible  view 
of  the  homologies  is  that  which 
derives  the  limb  of  the  higher  ver- 
tebrates from  one  like  that  of  the  ganoid  or  selachian  by  the 
loss  of  most  of  the  basals  and  radials,  a  single  basal  giving  rise 
to  the  humerus  or  femur,  the  proximal  portions  of  a  couple  of 
radials  forming  the  ulna  and  radius  (tibia  and  fibula),  while 
the  distal  portions  of  the  same  radials,  with  possibly  parts  of 
others,  producing  the  distal  parts  of  the  limb.  The  accom- 
panying diagram  illustrates  the  general  outlines  of  the  process, 


FIG.  189.  Diagram  illustra- 
ting possible  evolution  of  penta- 
dactyle  limb  from  the  ventral  fin 
of  fishes ;  the  shaded  portion 
represents  the  persistent  parts. 


SKELETON. 

those  parts  which  are  shaded  being  those  which  persist  in  the 
higher  vertebrates. 

In  the  fin  skeleton  of  the  elasmobranchs  two  basals,  an 
anterior  protopterygium  and  a  posterior  metapterygium,  occur  in 
the  ventral  fin,  while  in  the  pectoral  fin  a  third  basal,  the  mesop- 
terygium,  is  intercalated  between  the  other  two.  These  basals. 
support  a  richly  developed  radial  system,  the  radialia  being  with 
few  exceptions  developed  on  one  side  of  the  axis  formed  by  the 
basals.  In  the  lower  ganoids,  on  the  other  hand,  the  basals  are 
more  numerous,  and  show  the  primitive  conditions  more  clearly 
than  do  the  elasmobranchs.  In  both  of  these  groups  the  rays 
of  dermal  origin  are  well  developed,  and  reach  their  extreme  in 


FIG.  190.  Typical  pentadactyle  limbs ;  above  fore  limb,  below  hind  Iim&_ 
c,  centrale ;  cp,  carpus;  /,  femur;  fe,  fibulare  ;  Ji,  fibula;  h,  humerus;  t,  interme- 
dium; me,  metacarpals ;  ;///,  metatarsals ;  p' ,  p"  •>  phalanges;  r,  radius;  re,  radiale; 
/,  tarsus ;  te,  tibiale ;  //,  tibia ;  «,  ulna;  ue,  ulnare  ;  7-F,  digits;  1-5,  carpales  or 
tarsales. 

teleosts  in  which  the  cartilage  bones  of  the  fin  are  greatly  re- 
duced. In  the  pectoral  fin  they  are  represented  by  (usually) 
four  bones  (actinosts)  which  support  the  dermal  rays,  either 
directly  or  by  the  intervention  of  cartilaginous  radials.  In  the 
dipnoi  there  is  a  well-developed  and  segmented  axial  skeleton 
to  the  fin  which  may  be  without  other  skeletal  parts  {Protopterus, 
Fig.  269),  or  which  may  bear  biserial  lateral  branches  which 
connect  with  the  dermal  rays  (Ceratodus,  Fig.  185). 

Throughout  the  higher  vertebrates,  from  the  amphibia  to- 
man, the  same  type  of  limb  structure  is  everywhere  found,  that 


1/6     MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 

of  the  anterior  and  posterior  limbs  being  essentially  identical.  In 
the  fore  limb  there  is  in  the  region  corresponding  to  the  upper 
arm  (brachium)  of  man  a  single  bone,  the  humerus ;  in  the  fore 
arm  (antibrachium)  two  bones,  the  radius  on  the  anterior,  the 
ulna  on  the  posterior  side.  In  the  wrist  (carpus)  there  are 
typically  nine  small  bones,  arranged  in  three  series.  The  first 
consists  of  a  radiale  on  the  radial  side,  an  ulnare  on  the  ulnar 
side,  and  between  these  an  intermedium.  The  second  series 
consists  of  a  single  centrale ;  while  the  distal  series  is  a  row  of 
five  carpales,  numbered  from  one  to  five,  beginning  on  the  radial 
(thumb)  side.  In  the  hand  (manus)  are  recognized  metacarpus 
(palm)  and  digits  (fingers).  There  are  five  metacarpal  bones 
in  the  palm,  while  the  digits  are  composed  of  a  number  of  bones 
arranged  in  series  (phalanges).  The  fingers  are  numbered 
from  one  to  five,  beginning  at  the  radial  or  thumb  (pollex)  side. 

In  the  hind  limb  the  femur  corresponds  to  humerus  ;  tibia 
and  fibula  to  radius  and  ulna  respectively.  The  ankle  (tarsus) 
consists  of  tibiale,  fibulare,  intermedium,  centrale,  and  five  tar- 
sales,  and  these  are  succeeded  by  metatarsals  and  phalanges, 
which  are  numbered  from  one  to  five,  beginning  at  the  hallux 
(large  toe).1 

These  parts  can  be  greatly  modified,  the  chief  changes  con- 
sisting of  fusion  or  disappearance  of  some  of  these  elements. 
These  alterations  are  usually  more  marked  in  the  distal  por- 
tions, while  those  bones  nearer  the  body  are  less  subject  to 
modification.  Occasionally  bones  may  be  added  to  these  typi- 
cal ones ;  thus,  there  may  be  two  centrales,  and  again,  there 
may  be  membrane  (sesamoid)  bones  added  to  the  wrist  or 
ankle.  In  cases  where  the  details  of  reduction  can  be  clearly 
traced,  it  is  found  that  the  outer  digits  are  the  first  to  disap- 
pear, the  order  of  disappearance  usually  being  i,  5,  2,  4. 

In  human  anatomy  different  names  have  been  given  to  the 
carpal  and  tarsal  bones  from  those  employed  here ;  and  as  in 
the  older  works  this  nomenclature  has  been  transferred  to 
other  groups,  the  following  table,  which  shows  the  usually 
accepted  homologies,  may  prove  of  value. 

1  The  student  is  referred  to  special  works  for  a  discussion  of  those  cases,  like  the  frog 
and  the  mammal  Pedetes,  which  seem  to  indicate  the  existence  of  more  than  five  digits. 


CARPUS. 

radiale            =  scaphoid, 
intermedium  =  lunare. 

tibiale 
intermedium 

ulnare             —  cuneiforme. 

fibulare 

centrale          =  centrale. 

centrale 

carpale  l          —  trapezium. 
carpale2          =  trapezoid. 
carpale3          =  magnum. 

carpale*    )     ;=  unciform. 
carpale  5    ) 

tarsale  l 
tarsale  2 
tarsale  3 
tarsale  4 
tarsale  8 

TARSUS. 

=  astragulus. 

=  calcaneum. 

=  naviculare. 

=  internal  cuneiform. 

=  middle  cuneiform. 

=  external  cuneiform. 

cuboid. 


The  pisiform  of  the  carpus  is  a  sesamoid  bone  (i.e.,  a_inem- 
brane  bone  developed  in  a  tendon  as  a  result  of  strain  or  pres- 
sure), while  the  centrale  often  fuses  with  the  carpale  3  to  form 
the  os  magnum. 

The  median  or  unpaired  fins  which  develop  from  the  dorsal 
pair  of  lateral  folds  and  the  postanal  part  of  the  ventral  folds 
occur  only  in  the  ichthyopsida.  The 
result  of  such  a  union  of  folds  (p. 
172)  would  be  to  produce  a  fin  in  the 
median  line  which,  beginning  on  the 
back,  should  continue  around  the 
tail  and  forward  upon  the  ventral 
surface  as  far  as  the  vent.  Such  a 
continuous  fin  occurs  in  the  cyclo- 
stomes,  larval  amphibia,  and  many 
other  forms ;  but  usually  it  is  inter- 
rupted, and  thus  divided  into  dorsal, 
caudal,  and  anal  (on  the  ventral  sur- 
face) fins.  In  the  amphibia  these 
fins  are  without  skeletal  support ; 
but  in  the  fishes  a  regular  skeleton 
is  formed,  consisting  of  segmentally 
arranged  basalia  and  radialia  like 
those  of  the  primitive  paired  fins, 
and  besides,  a  system  of  dermal  fibrous  supports.  Occasionally, 
however,  there  is  to  be  found  an  intercalation  of  radialia,  these 
sometimes  being  at  least  twice  as  numerous  as  the  somites. 


FIG.  191.  Dorsal  vertebrae  of 
Pleuracanthus,  after  Fritsch.  <:, 
notochord;  /&,  haemal  arch;  «, 
neural  arch ;  r,  radialia  of  dorsal 
fin,  showing  intercalation  of  ele- 
ments. 


MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


ORGANS    OF    CIRCULATION. 

The  circulatory  structures  of  the  vertebrates  consist  of 
fluids  (blood  and  lymph)  and  the  vessels  in  which  they  flow, 
certain  parts  of  which  are  specialized  for  the  propulsion  of  the 
contained  fluid.  The  general  characters  of  the  blood  and  lymph 
have  already  been  described  ;  details  will  be  given  below  when 
necessary.  On  a  priori  grounds  the  lymph  system  is  apparently 
the  older,  but  it  will  be  more  convenient  to  begin  our  account 
with  the  blood-vascular  system.  In  this  we  recognize  in  all 
vertebrates  a  central  muscular  organ,  the  heart,  which  propels 
the  blood,  vessels  (arteries)  which  convey  the  blood  to  the  pe- 
ripheral portions,  and  other  canals  (veins)  which  bring  it  back 
to  the  heart ;  the  extremities*  of  the  arteries  and  veins  being 
connected  by  minute  tubes,  —  the  capillaries. 

There  is  considerable  evidence  to  support  the  view  of 
Biitschli,  that  the  main  trunks  of  the  circulatory  system  are 


FIG.  192.     Diagram  of  primitive  condition  of  blood-vessels,     a,  transverse 
vessels;  d  and  v,  dorsal  and  ventral  vessels. 

V^Gt     ^  / I)  (st  F\     ~* 

remnants  of  the  segmentation  cavity  (p.  5)  of  the  embryo, 
which  has  otherwise  become  entirely  obliterated  by  the  ingrow- 
ing mesoderm.  The  extension  of  the  coelomic  pouches  towards 
the  middle  line  of  the  body  above  and  below  the  alimentary 
tract  narrows  the  segmentation  cavity  in  these  regions  into  two 
longitudinal  tubes,  the  main  circulatory  trunks  ;  while  from  those 
portions  of  the  cavity  between  the  myotomes  arise  semicircular 
tubes  uniting  the  dorsal  and  ventral  tubes,  the  result  being  rep- 
resented diagrammatically  in  Fig.  192.  A  part  of  the  ventral 
tube  near  the  anterior  end  becomes  specialized  as  the  heart ;  it 
forces  the  blood  forward  through  the  anterior  end  of  the  ventral 
tube  which  is  known  as  the  ventral  aorta,  then  dorsally  through 


ORGANS   OF  CIRCULATION. 


179 


the  connecting  semicircular  vessels  (aortic  arches),  and  thence 
back  through  the  dorsal  tube  (dorsal  aorta)  to  again  enter  the 
posterior  (venous)  portion  of  the  ventral  tube,  and  thence  back 
to  the  heart. 

In  development  much  of  this  probable  ancestral  history  has 
been  masked.  Many  of  the  vessels  which  theoretically  should 
appear  as  spaces  between  the  myotomes  are  formed  as  solid  cords 
of  cells  (often  as  a  single  row  of  cells),  which  later  become 
canalized  and  converted  into  tubes.  We  may  first  describe  this 
system  of  circulatory  vessels  as  they  become  developed  in  the 
lower  vertebrates,  taking  them  up  in  the  order  —  heart,  arteries, 
and  veins,  and  then  trace  the  modifications  which  occur  in  the 
higher  groups.  This 
method  has  the  advantage, 
as  it  traces  the  ontogenetic 
steps  by  which  the  amniote 
circulation  arises. 

In  the  development  of 
the  heart  three  parts  are  to 
be  considered,  —  the  epi- 
thelium lining  it,  its  mus- 
cular walls,  and  the  cavity 
(pericardium)  in  which  it 
is  suspended. 

Just  behind  the  place 
where  the  first  (hyoman- 
dibular)  gill  slit  is  to  ap- 
pear, the  descending  edges 


FIG.  193.     Section  through  the  throat  region 
of  an  embryonic  Amblystoma,  illustrating  the 


Of    the    lateral    plates,    COr-      early  formation  of  the  heart,    e,  endothelium  of 
responding     in      length     tO      heart;  «•,  ectoderm;/,  fusion  of  ectoderm  and 


several  somites,  meet  just     ™tof  rm  thr°ugh  ^  the  jf11  cleft  wil1 

/  develop  later  from  the  gill  pouch,  g;  w,  myo- 

above  the  ventral  epider-  tome;  ms,  remains  of  ventral  mesocardium, 
mis,  while  more  dor  Sally  the  dorsal  mesocardium  has  not  yet  formed; 

they  enclose  a  groove-like     n>  notochord;  A  pericardial  wall;  pc,  peri- 

cardial  cavity  ;  s,  spinal  cord. 

space    open    to    the    yolk 

above.  In  this  groove  appear  cells  which  ultimately  develop 
into  the  epithelium  (endothelium)  of  the  heart  ;  but  the  origin 
of  these  cells  is  not  certainly  known.  The  evidence  tends  to 


180     MORPHOLOGY  OF   THE    ORGANS  OF   VERTEBRATES. 


show  that  they  are  derived  from  the  yolk  (entoderm)  ;  but  the 
investigation  is  difficult,  and  they  may  arise  from  the  mesothe- 
lium,  or,  less  probably,  they  may  be  mesenchymatous  in  origin. 
These  cells  arrange  themselves  into  a  tube  which  is  to  form  the 
lining  (epithelium)  of  the  heart  and  ventral  aorta,  while  behind 
the  heart  region  they  extend  backwards  as  a  pair  of  tubes,  the 
omphalomesaraic  veins  to  be  described  later,  on  either  side  of 

the  yolk.    . 

In  the  heart  region  the 
edges  of  the  lateral  plates 
now  fuse  in  the  median 
line  above  and  below  the 
endothelial  tube,  thus  giv- 
ing rise  to  two  longitu- 
dinal folds,  a  dorsal  and  a 
ventral  mesocardium ; 
while  that  part  of  the  lat- 
eral plates  surrounding 
the  endothelium  later  de- 
velops the  muscular  wall 
(myocardium)  of  the  heart 
and  the  ventral  aorta. 
The  downward  growth  of 
the  lateral  plates  brings 
the  ccelom  just  outside  the 
myocardium,  and  this  part 
of  the  ccelom  becomes  cut 
off  from  the  rest,  and 
gives  rise  to  a  space,  the 


FlG.  194.  Early  heart  of  Amblystoma,  after 
a  reconstruction  by  Dr.  F.  D.  Lambert,  a, 
auricle ;  b  *,  b  ^,  branchial  arches,  1-4 ;  dm, 
dorsal  mesocardium  j  A,  hyoid  arch;  m,  man- 
dibular  head  cavity;  o,  omphalomesaraic  veins; 
/,  pericardial  chamber ;  j,  sinus  venosus ;  /, 
truncus  arteriosus  ;  v,  ventricle;  I,  first  aortic 


arch.  pericardial  cavity,  sur- 

rounding the  heart. 

At  first  the  dorsal  and  ventral  mesocardia  are  entire,  and 
while  dividing  the  pericardial  space  into  right  and  left  halves, 
suspend  the  tube  in  this  chamber  in  the  same  way  that  the  in 
testine  is  supported  by  the  mesenteries  farther  back.  Soon  the 
ventral  mesocardium  breaks  down,  while  a  little  later  the  dorsal 
membrane  becomes  reduced  to  a  small  support  for  the  posterior 
portion  of  the  heart.  At  first  the  tube  is  straight,  and  equal  in 


ORGANS   OF  CIRCULATION. 


181 


length  to  the  pericardium  ;  but  it  rapidly  increases  in  length, 
and  as  a  result  becomes  twisted  into  an  S-shaped  tube,  and  with 
this  twisting  the  heart  becomes  differentiated.  In  the  S  are 
developed  two  chambers,  an  anterior  ventricle  and  a  posterior 
atrium  or  auricle,  the  tube  between  these  remaining  smaller,  — 
the  atrio-  or  auriculo-ventricular  canal.  In  this  twisting  only 
the  posterior  portion  of  the  tube  takes  part,  and  the  atrium  comes 
to  have  the  more  dorsal  position  (Fig.  194). 

The  anterior  straight  portion  of  the  tube  gives  rise  to  the 
truncus  arteriosus  and  the  ventral  aorta.  In  the  truncus  region, 
which  immediately  adjoins  the  heart,  two  parts  may  be  differen- 
tiated, —  a  posterior  conus  arteriosus,  containing  on  its  inside 
membranous  valves  preventing  any  backward  flow  of  the  blood, 
and  an  anterior  and  muscular  bulbus  arteriosus.  Behind  the  heart 
the  two  omphalomesaraic  veins  unite  to  form  a  cavity,  the  sinus 
venosus,  into  which,  later,  other  veins  entering  the  heart  come 
to  empty.  Valves  soon  arise  in  the  auriculo-ventricular  canal, 
and  a  little  later  other  valves  are  formed  at  the  opening  of 
the  sinus  into  the 
atrium.  These 
valves  are  fleshy 
folds  which  prevent 
any  backward  flow 
of  the  blood. 

In  its  earlier 
stages  the  heart  lies 
in  the  region  of  the 
gill  slits  (Fig.  122); 
but  as  the  animal 
increases  in  age 
there  is  a  relative 
shifting  of  parts,  so 
that  the  heart 
comes  to  lie  behind 
the  gills,  and  in 
many  forms  is  removed  some  distance  from  them. 

The  conditions  so  far  described  are  permanent  in  fishes,  and 
also  occur  in  the  younger  stages  of  -all  higher  forrrts,  with  the 


FIG.  195.  Early  stage  in  the  development  of  the 
heart  in  the  tern  {Sterna).  «,  anterior  end  of  the  ali- 
mentary canal ;  c,  coelom,  later  cut  off  as  pericardium ; 
e,  epidermis;  /<?,  left  endothelial  cavity;  n,  notochord; 
pi  wall  of  somatoplure,  which  later  gives  rise  to  muscles 
of  the  heart;  re,  right  half  of  heart;  so,  somatoplure; 
sp,  splanchnoplure;  vtn,  ventral  mesocardium. 


1 82      MORPHOLOGY  OF   THE   ORGAN'S  OF   VERTEBRATES. 


exception  of  one  feature  in  the 
amniotes.  In  these,  as  a  result 
of  the  great  size  of  the  yolk,  the 
heart  at  first  appears  as  a  pair  of 
widely  separated  tubes  (Fig.  ,195), 
which  later  approach  and  then 
unite  to  form  the  single  tube, 
which  then  undergoes  the  twist- 
ing and  differentiation  already 
described. 

The  ventral  aorta  extends  for- 
ward from  the  heart  beneath  the 
pharynx.  It  is  a  part  of  the  same 
primitive  tube  from  which  the 
heart  arises.  From  this  tube 
there  are  given  off  vessels  —  right 
and  left  —  (aortic  arches)  which 
pass  outward  in  the  tissue  between 
the  gill  slits,  then  up  on  either 
side  of  the  pharynx,  and  at  last 
those  of  each  side  unite  dorsally 
to  form  a  vessel  (radix  aortae) 
above  the  pharynx.  Behind  the 
region  of  the  gill  slits  the  radices 
of  the  two  sides  unite  to  form  a 
tube  (dorsal  aorta),  running  back- 
ward between  the  notochord  (ver- 
tebral column)  and  the  alimentary 
canal  to  the  end  of  the  body. 

From  these  arterial  vessels 
smaller  arteries  are  given  off  to 
FIG.  196.  Diagram  of  early  suppiy  the  various  regions  of  the 

arterial  circulation,     a,  auricle  ;  aa, 

anterior  abdominal  artery;   b,  bul-       **%'       P  rOm    the    first    °r   anterior 

bus  arteriosus;  c,  conus  arteriosus; 

ca,  caudal  artery;  ci,  common  iliac  artery;  col,  cceliac  axis;   <:/,  carotids;   d,  dorsal 

aorta;  /,  femoral  artery;  z,  iliac  artery;    z'/;/,  inferior  mesenteric  arteries ;  j,  jugular 

vein;  m,  metameric    (intercostal)  arteries;   o,  omphalomesaraic  (hepatic)  veins;/, 

postcardinal  veins;    ra,  radix  aortse;    s,  sinus  venosus  ;  sc,  subclavian  artery;   sm, 

superior  mesenteric  artery ;  v,  ventricle ;  va,  ventral  aorta ;  vt,  vertebral  artery. 


ORGANS  OF  CIRCULATION. 


183 


aortic  arch  of  either  side  arise  two  arteries,  the  external  (ven- 
tral) and  internal  (dorsal)  carotids,  which  run  forward  to  supply 
the  head  ;  the  external  being  distributed  to  the  muscles  of  the 
head  and  tongue,  the  internal  going  up  through  the  floor  of  the 
skull  to  the  brain. 

Farther  back  the  dorsal  aorta  gives  off  vessels,  right  and 
left,  to  the  adjacent  seg.ments  (see  p.  188),  and  then  gives  off 
two  larger  trunks,  the  omphalomesaraic  arteries,  which  at  first 
connect  directly  with  the  omphalomesaraic  veins  already  men- 
tioned. Behind  the  origin  of  these  arteries  the  dorsal  aorta  is 
a  paired  structure,  but  soon  the  two  halves  unite  into  the  single 
vessel  found  in  the  adult  of  all  forms.  Near  the  posterior  end 


FlG.  197.  Diagram  of  early  circulation  in  a  vertebrate  with  small  yolk.  DC, 
ductus  Cuvierii;  EC,  external  carotid;  H,  heart;  HA,  hypogastric  artery;  1C, 
internal  carotid;  J,  jugular  vein;  L,  liver;  M,  point  of  formation  of  mouth; 
OA,  omphalomesaraic  artery ;  O  V,  omphalomesaraic  vein  ;  PC,  posterior  cardinal 
vein  ;  V,  vent. 

of  the  peritoneal  cavity  the  dorsal  aorta  gives  off  a  pair  of  hypo- 
gastric  arteries  which  run  downward  on  the  side  of  the  alimen- 
tary canal,  and  behind  these  the  aorta  continues  into  the  tail  as 
the  caudal  artery. 

A  little  later  another  system  of  veins  arises.  These  are  the 
jugulars,  or  anterior  cardinals,  and  the  posterior  cardinals.  These 
run  on  either  side  of  and  a  little  below  the  backbone,  the  jugu- 
lars coming  from  the  head,  the  posterior  cardinals  from  the  dorsal 
wall  of  the  body  cavity.  These  vessels  of  either  side  unite  just 
above  the  heart  into  a  transverse  vessel,  the  ductus  Cuvierii, 
which  empties  into  the  sinus  venosus. 

The  foregoing  gives  in  outline  the  great  vascular  trunks  of 
the  body  ;  but  these  undergo  many  modifications  in  the  different 


1 84     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

groups  of  vertebrates,  while  other  vessels,  both  veins  and  arte- 
ries, are  developed  to  supply  the  various  organs.  The  alterations 
from  these  outlines  are  now  to  be  traced. 

Heart.  —  The  heart  always  retains  its  primitive  twist,  and 
the  atrium  and  its  derivatives  lie  dorsal  to  or  even  in  front  of 
the  ventricular  portion.  In  the  lower  fish-like  forms  the  heart 
of  the  adult  can  be  reduced  to  the  condition  outlined  above,  but 
from  the  dipnoi  and  amphibia  upwards  (that  is,  with  the  appear- 
ance of  lungs)  this  organ  is  more  or  less  completely  divided  into 
right  and  left  halves  by  a  vertical  septum  which  grows  from 
behind  forwards.  In  the  groups  just  mentioned  this  septum 
divides  the  atrium  into  right  and  left  halves,  —  the  auricles  of  the 
heart.  The  sinus  venosus  retains  its  connection  with  the  right 
auricle,  while  the  pulmonary  vein,  bringing  blood  from  the  lungs, 
empties  into  the  other.  Thus  the  two  auricles  receive  different 
kinds  of  blood.  That  which  enters  by  way  of  the  sinus  comes 
from  all  parts  of  the  body,  and  is  consequently  poor  in  oxygen 
and  contains  much  carbon  dioxide  ;  while  that  coming  from  the 
lungs  is  rich  in  oxygen  and  lacking  in  carbon  dioxide.  These 
two  kinds  of  blood  are  known  respectively  as  venous  and 
arterial. 

In  the  contraction  of  the  auricles  the  blood  from  the  left 
auricle  is  first  forced  into  the  ventricle,  while  the  venous  blood 
follows  later,  and  thus  comes  to  occupy  the  posterior  portion  of 
the  ventricle.  These  two  kinds  of  blood  are  now  forced  through 
different  portions  of  the  aortic  arches.  In  the  truncus  arteriosus 
a  spiral  fold  or  valve  occurs,  extending  as  far  forward  as  the  pos- 
terior aortic  .arch.  When  the  ventricle  contracts,  this  blood  at 
first  flows  forward  in  the  ventral  aorta  into  the  anterior  aortic 
arches,  which  consequently  receive  arterial  blood.  As  the  aorta 
fills,  the  spiral  valve  moves  in  such  a  way  that  the  venous  blood 
flows  into  the  posterior  arches.  In  the  reptiles  the  cardiac 
septum  extends  into  the  ventricle,  dividing  it  partially  or  com- 
pletely (crocodiles)  into  right  and  left  halves.  Even  in  those 
cases  where  the  septum  between  the  two  halves  is  incomplete, 
there  is  a  physiological  division,  for  at  the  time  of  contraction 
the  walls  and  the  septum  come  together  so  as  to  separate  the 
two  sides.  In  the  birds  and  mammals  the  separation  is  com- 


ORGANS  OF  CIRCULATION. 


I85 


plete.     Hence  in  all  amniotes  we  can  recognize  an  arterial  (left) 
and  a  venous  (right)  side  to  the  heart,  each  side  consisting  of 

practically  an  auricle  and  a  ventricle. 
In  the  mammals  and  birds  the  divis- 
ion also  extends  to  the  truncus  as 
far  forward  as  the  first  aortic  arch,  so 
that  these  vessels  are  connected  with 
the  right  auricle,  the  other  arches 
being  connected  with  the  arterial 
half  of  the  heart.  In  the  reptiles 
the  division  is  carried  farther  ;  for  the 
fourth  arch  of  the  left  side  has  its 
own  trunk,  and  this  is  connected 
with  the  right  side  of  the  heart.  The 
effect  of  this  will  be  apparent  after 
we  consider  the  aortic  arches. 

Aortic  Arches. —  In  all  vertebrates 
except  the  cyclostomes  and  lower 
sharks  the  aortic  arches  are  typically 
five  in  number  ; 1  but  in  all  except 
the  elasmobranchs  the  number  is  re- 
duced by  the  disappearance  of  the 
second  normal  arch.  In  the  follow- 
ing the  arches  are  numbered  one  to 


FIG.  198.  Diagram  of  the 
heart  and  aortic  arches  of  the 
alligator,  after  Hertwig.  fa, 
left  auricle ;  lao,  left  aortic 
arch  ;  !av,  left  auriculo-ventric- 
ular  aperture ;  Ic,  left  carotid  ; 

//,  left  pulmonary  artery;  is,  &VQ,  although  that  number  may  not 
left  subciavian;  /<•/,  left  ven-  be  actually  present.  In  the  ichthy- 
tricie.  The  right  side  with  cor-  opsjda  these  arches  really  consist  of 

responding  letters. 

two  parts,  one  arising  from  the  ven- 
tral aorta,  the  others  connecting  with  the  dorsal  aorta.  In  the 
gill  arches  these  two  vessels  run  parallel  to  each  other,  the  con- 
nection between  them  being  effected  by  capillary  loops  which 
run  through  the  external  or  internal  gill  filaments.  In  passing 
through  these  gills  the  blood  loses  its  carbon  dioxide  and  takes 
up  oxygen,  and  thus  enters  the  dorsal  aorta  as  arterial  blood.  In 
dipnoi,  amphibia,  and  higher  groups,  in  which  lungs  appear,  the 
posterior  (fifth)  arch  of  either  side  sends  a  branch,  the  pulmo- 


1  There  is  some  evidence  to  show  that  the  number  is  really  six,  an  arch  dropping  out 
between  the  fourth  and  fifth  of  those  recognized  here. 


1 86     MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 

nary  artery,  back  into  the  corresponding  lung.  With  the  loss 
of  gills  in  the  amphibia  and  their  absence  in  the  higher  groups, 
the  lungs  become  the  chief  respiratory  organs,  and  the  proximal 


FIG.  199.  Aortic  arches  of  Amblystoma  embryo,  after  a  reconstruction  by 
Dr.  F.  D.  Lambert,  aa  1~6,  afferent  arteries ;  b,  balancer ;  ba,  extremity  of  bul- 
bus  arteriosus ;  c,  common  carotid ;  da,  dorsal  aorta ;  e  8-5,  efferent  arteries ;  h, 
hyoid  arch  ;/,  jugular  vein;  ra,  radix  aortse;  v,  vein  from  balancer  to  jugular;  va, 
ventral  aorta ;  1-5,  places  where  gill  clefts  are  to  form.  Notice  that  the  second 
aortic  arch  is  lacking. 


ORGANS  OF  CIRCULATION. 


I87 


portion  of  the  arch  and  the  pulmonary  artery  increase  in  size, 
while  that  portion  of  the  arch  between  the  pulmonary  artery 
and  the  radix  remains  undeveloped  (ductus  Botalii)  or  becomes 
entirely  aborted.  In  the  amphibia  also  the  radix  disappears  be- 
tween the  first  and  third  arches,  so  that  blood  forced  through 
the  anterior  arches  can  only  go  to  the  head  through  the  carotids. 


MAMS. 


FIG.  200.  Diagrams  of  the  aortic  arches  in  different  groups  of  vertebrates. 
A,  fishes;  B,  amphibia;  C,  reptiles;  D,  birds;  E,  mammals;  a,  ventral  aorta; 
c,  internal  carotids ;  e,  external  carotids  ;  /,  pulmonary  arteries  ;  s,  subclavian 
arteries.  After  Lambert. 

In  the  urodeles  two  arches  on  either  side  (three  and  four)  con- 
nect with  the  dorsal  aorta  by  means  of  the  radices  ;  but  in  the 
anura,  on  the  assumption  of  the  adult  condition,  the  fourth  arch 
degenerates. 

In  the  reptiles  the  conditions  are  somewhat  complex.  In 
certain  lizards  the  third  arch  as  well  as  the  fourth  may  be  con- 


1 88     MORPHOLOGY  OF  THE    ORGANS  OF   VERTEBRATES. 


'nected  with  the  radix,  so  that  both  arches  are  aortic  in  character, 
while  the  blood  going  through  the  third  arch  in  part  goes  to 
the  carotids.  In  the  other  reptiles  the  connection  between  the 
third  and  fourth  arches  disappears  (Fig.  200  £7),  and  here  the 
third  arch  is  purely  carotid  in  character.  The  right  arch  of 
the  fourth  pair  forms  the  main  trunk  of  the  dorsal  aorta,  while 
the  left,  which  as  we  have  seen  is  connected  with  the  right  side 
of  the  heart,  is  largely  distributed  to  the 
digestive  organs,  only  connecting  with  the 
dorsal  aorta  by  a  small  trunk  (Fig.  198). 
As  a  result  of  this  distribution  of  vessels 
the  carotids  and  the  dorsal  aorta  receive 
arterial  blood,  while  the  stomach  receives 
only  venous  or  mixed  blood.  The  re- 
mainder of  the  venous  blood  goes  through 
the  fifth  arch  to  the  lungs. 

In  the  birds  and  mammals  where  there 
\  are  but  two  arterial  trunks,  all  of  the  venous 
blood  goes  to  the  lungs,  all  parts  of  the 
systemic  arteries  receiving  arterial  blood. 
The  chief  distinction  between  these  groups 
lies  in  the  fact  that  in  the  birds  the  right 
half,  in  mammals  the  left,  of  the  fourth 
arch  persists  (Fig.  200  D  and  E). 

Arteries.  —  As  development  proceeds 
other  arteries  than  those  mentioned  on 
p.  183  arise  from  the  dorsal  aorta  and  its 
radices.  The  chief  of  these  are  the  follow- 
ing. In  the  cervical  and  posterior  cranial 
regions  are  as  many  segmental  arteries  as 
there  are  segments.  These  are  united  by 
anastomoses  on  either  side,  after  which  the 
roots  of  the  segmental  arteries  themselves  disappear,  with  the 
exception  of  the  last,  which  remains  as  the  stem,  while  the 
anastomosing  vessel  on  either  side  persists  as  the  vertebral  ar- 
tery, growing  forward  into  the  head,  where  it  anastomoses  with 
the  carotids.  From  the  stem  of  the  vertebral  artery  the  sub- 
clavian  artery  arises  as  a  bud,  and  with  the  outgrowth  of  the 


FlG.  201.  Diagram 
of  the  circulation  in  a 
mammal,  the  arterial 
parts  white,  the  venous 
shaded ;  the  arrows 
show  the  direction  of 
the  flow  of  blood,  a, 
auricles ;  /,  lung ;  lu, 
liver ;  /,  portal  vein ; 
•v,  veins. 


ORGANS   OF  CIRCULATION. 


189, 


limb  it  extends  into  that  member.1     In 
the  higher  vertebrates  the  subclavian, 
on  entering  the  fore  limb,  is  known  as 
the    axillary    artery,    and 
farther      down     as     the 
brachial    artery,    the 
brachial    dividing    in 
the  fore  arm  into  ra- 
dial     and     ulnar 
branches    which    run 
near  the  correspond- 
ing bones. 

With  the  absorp- 
tion of  the  yolk,  the 
omphalomesaraic    ar- 
teries   undergo 
changes.     They  early 
lose  their  direct  con- 
nection with  the  omphalo- 
mesaric  veins  (Fig.   197)* 
while  that  of  the  left  side 
disappears    without    trace 
that  of  the  right  carrying 
blood    to   the   yolk,   which 
finds   its  way  to    the   om- 
phalomesaraic   vein    by    a  # 
system  of  yolk  capillaries. 
From   this   persistent  ves- 
sel   a    branch    grows    out 


1  Typically  the  subclavians  arise 
from  the  radix  of  either  side ;  but  they 
may  have  their  origin  in  the  adult  from 
the  dorsal  aorta,  or,  exceptionally,  right 
and  left  subclavians  may  be  given  off 
from  the  radix  of  one  side. 


Czt 


Act 


FIG.  202.  Arterial  system  of  Salamandrar 
from  Wiedersheim.  A,  allantoic  ;  Ao,  aorta  ; 
Bl,  bladder  ;  67,  cloaca ;  Cm,  mesenteric ;  Cr, 

crural ;  Cu,  cutaneous ;  d,  intestine ;  E,  epigastric  ;  <?,  rectum  ;  //,  hepatic ;  Hy, 
hypogastric  ;  /,  intestinal;  7/V,  common  iliac;  /,  liver;  Af,  rectal  arteries;  m, 
stomach ;  Ov,  genital  arteries  ;  T7,  arteries  to  pharynx  and  oesophagus  ;  A',  renal 
arteries  ;  RA,  radices  aortse  ;  Sc~,  subclavian. 


MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 

through  the  mesentery  to  the  anterior  end  of  the  intestine,  and 
becomes  the  superior  mesenteric  artery  of  the  adult. 

Anterior  to  the  superior  mesenteric  arise  arteries  supplying 
the  stomach,  spleen,  liver,  pancreas,  and  duodenum  (gastric, 
splenic,  hepatic,  and  duodenal).  In  the  lower  vertebrates  these 
are  more  or  less  distinct  in  their  origin  from  the  aorta,  while  in 
the  adults  of  the  higher  groups  they  unite  at  their  base  into  a 
common  trunk,  the  coeliac  axis ;  and  occasionally  the  superior 
mesenteric  may  fuse  for  a  short  distance  with  this,  forming  a 
coeliac-mesenteric  trunk. 

Between  the  superior  mesenteric  (omphalomesaraic)  and  the 
hypogastric,  other  arteries  (inferior  mesenteries)  supply  the 
hinder  end  of  the  intestine ;  and  these  again  may  arise  sepa- 
rately from  the  aorta,  or  their  roots  many  fuse  into  one  or 
more  trunks. 

The  hypogastric  arteries  (allantoics  of  amniotes)  are  rather 
more  complicated  in  their  relations.  In  the  fishes  they  are  dis- 
tributed on  the  ventral  wall  of  the  body  in  front  of  the  vent  and 
to  the  rectal  region  of  the  alimentary  tract,  while  branches,  the 
iliac  arteries,  are  given  off  from  each  to  the  pelvic  appendages. 
In  the  amphibia,  with  the  development  of  an  allantoic  diverticu- 
lum  from  this  region  which  later  forms  the  urinary  bladder, 
the  vessels  going  to  the  rectal  region  enlarge,  and  are  known  as 
the  allantoic  arteries.  In  the  embryonic  sauropsida  and  mam- 
mals the  allantois  becomes  greatly  developed,  and  grows  out 
(sauropsida)  into  close  connection  with  the  egg  shell,  or  (mam- 
mals) forms  the  placenta  (see  Mammalia)  which  enters  into 
intimate  relations  with  the  uterine  walls  of  the  mother.  Thus 
in  the  amniotes  the  allantois  becomes  an  important  organ  of 
respiration,  and  in  mammals  of  nutrition,  and  the  arteries  which 
reach  this  distal  portion  through  the  umbilicus  become  very 
important.  After  birth  (hatching)  respiration  and  nutrition 
are  accomplished  in  other  ways,  and  these  allantoic  vessels  con- 
sequently degenerate. 

With  the  appearance  of  legs,  the  iliac  arteries  increase  in 
importance.  Of  these  there  are  two,  —  an  anterior  external  and 
a  more  posterior  internal,  the  latter  arising  from  the  hypogastric 
trunk.  External  and  internal  iliacs  many  arise  separately  from 


ORGANS   OF  CIRCULATION.  19 1 

the  dorsal  aorta,  or  they  may  leave  it  as  a  single  trunk,  —  the 
common  iliac.  From  the  internal  iliac,  arteries  arise  to  supply 
the  various  viscera  of  the  pelvis,  and  also  an  ischiatic  or  sciatic 
artery,  which  passes  out  to  the  dorsal  portion  of  the  hind  limb, 
and  early  forms  the  chief  supply  of  this  appendage.  This  condi- 
tion persists  in  all  vertebrates  except  the  mammals.  In  these 
the  external  iliac  (after  its  entrance  into  the  limb  known  as  the 


FIG.  203.  Diagram  of  the  chief  circulatory  vessels  in  an  embryonic  sauropsidan. 
The  amnion  omitted  for  clearness.  A,  allantois;  AA,  allantoic  artery;  C,  carotid 
arteries;  CA,  caudal  artery;  CV,  caudal  vein;  DA,  dorsal  aorta;  DC,  ductus 
Cuvierii  ;  H,  heart ;  HA,  hypogastric  artery ;  L,  liver  ;  OA,  omphalomesaraic 
artery;  OV,  omphalomesaraic  vein ;  57",  sinus  terminalis  ;  UV*  umbilical  (allan- 
toic) vein;  V,  vent  ;  W,  vitelline  vein.  Compare  with  Fig.  197. 

femoral  artery)  extends  farther  into  the  limb,  and  usurps  the 
function  of  the  ischiatic,  which  here  supplies  only  the  posterior 
proximal  portion  of  the  appendage.  The  femoral  artery  extends 
down  into  the  bend  of  the  knee,  where  it  is  known  as  the  pop- 
liteal artery,  and  in  the  proximal  end  of  the  shank  divides  into 
an  anterior  tibial  artery  which  runs  along  the  anterior  face  of  the 
limb,  and  a  posterior  tibial  and  a  peroneal  in  the  calf  of  the  leg. 


MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

The  arteries  already  described  extend  ventrally  from  the 
aorta ;  but  there  arise  earlier  numerous  pairs  of  vessels,  seg- 
mentally  arranged  (see  p.  188),  which  run  out  in  a  transverse 
direction  to  the  muscles  and  to  the  urogenital  structures. 
Those  which  run  out  in  the  body  wall  to  supply  the  muscles, 
etc.,  are  known,  according  to  their  position,  as  the  intercostal 
and  lumbar  arteries  ;  those  going  to  the  excretory  organs  are  the 
renal  arteries ;  while  the  genital  arteries  (ovarian  or  spermatic)  go 
to  the  reproductive  organs  (gonads).  In  the  lower  vertebrates, 
where  the  pro-  or  mesonephros  is  functional  throughout  life,  the 
renal  arteries  retain  their  metameric  character  ;  but  with  the 
formation  of  a  metanephros  (amniotes)  the  segmental  arrange- 
ment is  lost,  and  the  kidneys  receive  their  blood  through  a  single 
pair  of  renal  arteries.  The  blood-vessels  supplying  the  gonads 
undergo  a  similar  reduction  in  the  higher  vertebrates. 

Veins.  —  The  primary  venous  trunks  have  been  enumerated 
on  a  previous  page  ;  they  are  a  pair  each  of  omphalomesaraics, 
jugulars,  and  posterior  cardinals. 

The  omphalomesaraic  veins  are  the  paired  posterior  continu- 
ations of  the  grooves  from  which  the  heart  is  formed.  They  con- 
tinue themselves  backwards,  and  at  first  are  connected  with  the 
omphalomesaraic  arteries  (see  Fig.  197).  Soon  this  connection 
is  lost,  and  the  vein  of  the  right  side  partially  disappears,  while 
the  other  sends  out  branches,  right  and  left,  over  the  yolk.  In 
those  vertebrates  which,  like  the  sauropsida,  have  a  large  yolk, 
these  vitelline  veins  play  an  important  part  in  the  early  develop- 
ment, but  with  the  absorption  of  the  yolk  they  disappear.  From 
the  point  where  the  vitelline  veins  arise  from  the  persistent  om- 
phalomesaraic, two  veins  grow  back  along  the  tail  beneath  the 
caudal  artery,  passing  on  either  side  of  the  rectum.  Fusion  of 
these  vessels  occurs,  and  there  results  a  single  caudal  vein  with 
a  loop  around  the  vent  (Fig.  205). 

In  the  beginning  the  two  omphalomesaraics  pass  on  either 
side  of  the  liver  ;  but  as  this  organ  develops,  the  left  omphalo- 
mesaraic sends  a  branch  into  it  from  behind,  while  from  the 
anterior  side  both  omphalomesaraics  extend  into  this  gland. 
There  is  thus  inaugurated  a  system  of  circulation  (the  portal 
system)  through  the  liver,  while  that  part  of  the  left  omphalo- 


ORGANS  OF  CIRCULATION, 


193 


mesaraic  which  passed  around  the  liver  degenerates,  and  the 
anterior  portions  of  these  veins  become  converted  into  the  he- 
patic veins,  conveying  blood  from  the  liver  to  the  sinus  venosus. 
In  this  process  that  portion  of  the  omphalomesaraic  vein  between 
the  liver  and  the  origin  of  the  vitelline  vein  becomes  twisted,  so 
as  to  surround  the  intestine  in  a  spiral  manner ;  and  this  portion 


FlG.  204.  Scheme  of  the  circulation  in  a  chick  of  the  third  day  from  below, 
after  Balfour  from  Wiedersheim.  AA,  aortic  arches;  Ao,  dorsal  aorta*;  DC,  ductus 
Cuvierii;  f/,  heart;  Lof,  left  omphalomesaraic  vein;  LofA,  left  omphalomesaraic 
artery;  ROf,  ROfA,  right  omphalomesaraic  vein  and  artery;  SCaV,  anterior 
cardinal  vein;  SV^  sinus  venosus;  ST,  sinus  terminalis;  VCa,  posterior  cardinal 
vein. 

persists  throughout  life  as  the  portal  vein  which  brings  blood 
from  the  intestine  to  the  liver,  while  that  part  of  the  caudal 
vein  which  lies  in  the  intestinal  region  develops  into  the  sub- 
intestinal  vein  of  the  adult.  The  fate  of  the  posterior  part  of 
the  caudal  vein  will  be  given  below. 

The  posterior  cardinals  at  first  extend  back  to  the  anterior 


194     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

ends  of  the  pronephros,  from  which  they  return  the  blood  to  the 
heart.  With  the  development  of  the  mesonephros  they  extend 
farther  back  until  they  reach  the  posterior  limits  of  the  abdomi- 
nal cavity.  Each  gives  off  intersegmental  veins  to  bring  back 
a  portion  of  the  blood  sent  into  the  abdominal  walls  by  the 
intercostal  and  lumbar  arteries,  while  in  the  early  stages  the 


IV 


Vlf 


VIII 


FIG.  205.  Development  of  the  venous  system  of  selachians,  after  Rabl  and 
Hochstetter.  C,  caudal;  CS,  cardinal  sinus;  J,  jugular;  LO,  left  omphalomes- 
araic;  /^portal;  PC,  postcardinal ;  RO,  right  omphalomesaraic ;  S,  sinus  venosus ; 
SI,  subinlestinal ;  V,  vitelline  (in  V  and  VII,  cloacal  loop).  Compare  VII  and 
VIII  with  Fig.  206. 

veins    from    both    extremities  (subclavians    and    hypogastrics) 
empty  into  the  same  vessels. 

These  post  cardinals  gradually  develop  a  rich  vascular  plexus 
in  the  mesonephros,  receiving  the  blood  brought  from  the  tail  by 
the  caudal  vein,  which  runs  forward  between  the  two  Wolffian 
bodies.  When  this  system  is  established  the  connection  between 


ORGANS   OF  CIRCULATION. 


195 


the  caudal  and  subintestinal  veins 
is  lost.  Modifications  now  set  up 
in  the  capillary  system  of  the  meso- 
nephros  not  easily  described  in  a 
few  words,  but  readily  made  out 
from  Fig.  205,  the  result  being 
that  the  interrenal  part  of  the  cau- 
dal vein  becomes  continuous  with 
both  posterior  cardinals,  while  the 
posterior  portion  of  the  caudal  vein 
divides  and  extends  forwards  upon 
the  lateral  sides  of  the  Wolffian 
bodies,  absorbing  the  posterior 
part  of  the  posterior  cardinals,  and 
with  them  receiving  the  blood 
from  the  posterior  appendages.1 

In  front  of  the  kidneys  in  elas- 
mobranchs,  the  posterior  cardinals 
meet  and  fuse  in  the  median  line, 
thus  forming  a  cardinal  sinus  be- 
tween the  gonads.  In  the  teleosts 
the  cardinals  unite  behind  with  the 
caudal  vein,  and  then  that  of  the 
left  side  closes  near  the  middle,  so 
that  the  blood  from  the  left  kidney 
usually  passes  backwards  to  enter 
the  right  cardinal  on  its  way  to  the 
heart. 

In  the  dipnoi  and  amphibia  a 
new  vein,  the  postcava  (vena  cava 
inferior),  comes  into  relations  with 
the  system  just  described.  It  be- 
gins as  an  outgrowth  of  the  right 
hepatic  vein,  and  extends  back  and 
joins  the  right  posterior  cardinal 
just  in  front  of  the  Wolfrian  body, 
so  that  now  a  large  part  of  the  renal 


FlG.  206.  Diagram  of  the 
venous  system  of  an  Amphibian. 
a,  auricle ;  aa,  anterior  abdominal 
vein ;  c,  caudal  vein ;  dc,  ductus 
Cuvierii ;  //,  hepatic  vein ;  ?,  iliac 
vein  ;  ir,  interrenal  vein  ;  _/,  jugu- 
lar ;  Ic,  left  cardinal;  n,  rheso- 
nephros;  p,  portal  vein;  pc, 
postcava ;  re,  right  cardinal ;  s, 
sinus  venosus ;  v,  ventricle. 


1  This  has  been  greatly  abbreviated  in  mammals. 


196     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


portal  blood  returns  to  the  heart  by  way  of 
the  postcava,  while  the  posterior  cardinals 
merely  receive  that  which  comes  back  from 
the  body  walls  by  way  of  the  intersegmental 
veins. 

In  the  amniotes,  a  renal  portal  system 
never  reaches  that  development  seen  in  the 
ichthyopsida,  and  is  only  found  in  connec- 
tion with  the  Wolffian  body,  i.e.,  in  embry- 
onic life.  When  first  formed,  blood  is 
returned  from  the  kidney  by  the  posterior 
cardinal  veins  ;  and  these  extend  back  and 
receive  the  iliac  veins  as  well,  no  interrenal 
vein  being  formed  by  the  co-operation  of 
caudal  and  postcardinal  veins.  When  the 
postcava  extends  back  as  far  as  the  per- 
manent kidneys,  it  sends  a  renal  vein  to 
each  ;  and  from  this  point  backwards  it  ab- 
sorbs the  right  postcardinal,  while  on  the 
other  side  lateral  veins  extend  out  and  take 
the  blood  formerly  brought  to  the  postcar- 
dinal of  the  left  side  by  the  intersegmental 

FIG.    207.      Venous     veins,    Fig.   2O8  C. 
trunks  of  man.     a,  azy-  .  .  . 

gos;  ,,  caudal  (sacralis  Farther  in  front  a  transverse  vein  arises 
media);  «,  common  from  the  right  postcardinal,  and  crosses 
iliac  ;«,  external  iliac;  over  and  unites  with  the  left,  which  now 

f   -ternal  Jugular;/,     }  .         connection    with     the    ductUS    Cu- 

femoral ;  g,  genital ;  ha, 

hemiazygos  (azygos  vierii  and  becomes  the  hemiazygos  vein,  the 
blood  from  which  now  passes  across  into 
the  right  post  cardinal,  called  in  man  the 
azygos  or  azygos  major.  In  this  way  all  of 

innominate;    /,     pre-    the  blood  from  the  hinder  half  of  the  body 

cava;  //;,  phrenic ;/.,  ,•  frQm  the  kidneys  and  behind)  flows 
postcava;  r,  renal;  ri,  ^ 

right  innominate;  back  to  the  heart  through  the  postcava. 
sft  subciavian ;  sr,  supra-  The  anterior  portion  of  the  left  posterior 
renal;  si,  superior  in-  cardinai  may  retain  its  connection  with  the 

U-rcostal ;  /,  thyroid.  .  .  ,    r     •  •  %      r     i 

anterior  veins  (lett  innominate)  or  the  same 
side,  and  become  converted  into  a  superior  intercostal  vein. 


minor);  ky ,  hypo  - 
gastric;  ii,  internal 
iliac;  (/,  internal  jugu- 
lar; >£,  kidney;  /*',  left 


ORGANS   OF  CIRCULATION.  1 97 

The  blood  distributed  by  the  hypogastric  arteries  is  returned 
to  the  heart  by  the  derivatives  of  a  pair  of  hypogastric  veins 
which  run  on  the  ventral  body  wall  forward  to  the  omphalomes- 
araic  vein.  When  the  hind  limbs  appear,  external  and  internal 
iliac  veins  grow  out  from  the  hypogastrics  into  those  appendages, 
their  ultimate  distribution  coinciding  more  or  less  closely  with 
the  similarly  named  arteries.  When  the  posterior  cardinals  grow 
back  into  this  region  they  tap  these  vessels,  and  so  the  blood 
from  the  hinder  appendages  is  returned  to  the  heart  through 
them,  at  first  directly,  later  through  the  renal  portal  system 
(Fig.  206),  and  in  the  higher  vertebrates  by  way  of  the  post- 
cava.  The  ventral  portions  of  the  hypogastric  veins  retain  their 
connection  with  the  iliacs  throughout  life  in  the  ichthyopsida 
(Fig.  206),  and  either  as  two  vessels  or  as  a  single  anterior 
abdominal  vein,  run  forward  in  the  ventral  body  wall,  and  enter 
the  portal  system  (Fig.  203). 

In  the  amniotes,  with  the  formation  of  the  allantois,  the  hy- 
pogastric veins  grow  out  into  this,  and  are  here  known  as  the 
umbilical  veins.  In  the  reptiles  they  retain  their  distinctness  ; 
but  in  birds  and  mammals  one  aborts,  leaving  the  other  as  a 
single  trunk  which  empties  into  the  omphalomesaraic.  Dur- 
ing embryonic  life  this  system  is  very  large  and  important,  but 
after  hatching  or  birth  it  becomes  reduced  to  an  inconspicuous 
condition,  Fig.  203. 

In  the  fishes  the  relations  of  jugulars  and  the  ducts  of 
Cuvier  are  much  as  outlined  above,  with  the  exception  that  the 
jugular  veins  develop  two  branches,  internal  and  external.  With 
the  formation  of  lungs  (dipnoi  and  amphibia)  this  system  be- 
comes unsymmetrical,  in  that  the  left  Cuverian  duct  is  now 
compelled  to  reach  the  right  side  of  the  sinus  venosus  ;  and  here, 
as  in  the  higher  groups,  the  trunks,  formed  of  united  jugulars, 
subclavians,  and  posterior  cardinals  (i.e.,  the  Cuverian  ducts), 
are  known  as  the  precavae,  right  and  left.  Here,  too,  is  to  be 
noticed  a  shifting  of  the  veins  (subclavians)  coming  from  the 
fore  limbs.  At  first  they  empty  into  the  posterior  cardinals,  but 
later  they  empty  into  the  jugulars,  the  common  trunks  formed 
by  the  subclavians  and  jugulars  being  known  as  the  innominate 
veins.  In  the  birds  a  transverse  anastomosis  forms  between  the 


198      MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 

jugulars  of  the  two  sides.  In  the  mammals  (Fig.  208)  a  trans- 
verse connection  forms  between  the  precavae  of  the  two  sides ; 
and  then  the  direct  connection  of  the  left  precava  with  the  heart 
is  lost,  so  that  all  the  blood  from  the  right  side  of  the  head  and 


FIG.  208.  Diagram  (altered  from  Gegenbaur)  showing  the  modifications  of 
the  venous  trunks  in  mammals,  tf ,  azygos  vein ;  ct  coronary ;  d,  ductus  Cuvierii ; 
eit  external  iliac;  ej,  external  jugular;  //,  hepatic;  ha,  hemiazygos ;  ic,  intercostals; 
zY,  internal  iliacs;  //,  internal  jugulars;  //,  left  innominate;  Ic,  left  posterior  cardi- 
nal; /,  precava;  po,  postcava;  r,  renal;  ;r,  right  posterior  cardinal;  ri,  right 
innominate;  s,  sinus;  sc,  subclavian ;  si,  superior  intercostal.  In  B  the  postcava 
has  extended  backwards  and  tapped  the  right  posterior  cardinal ;  and  a  transverse 
trunk  has  formed  between  the  jugulars  of  the  two  sides.  In  Ca  transverse  vessel, 
/,  has  united  the  two  postcardinals ;  and  these  have  lost  their  other  connections,  and 
form  the  azygos  system. 

the  right  fore  limb  passes  through  the  left  precava  in  its  way  to 
the  heart. 

The  lymph  system  forms  another  series  of  circulatory  vessels 
which  are  distinct  from  the  blood-vessels,  excepting  at  one  or 
more  points  where  they  connect,  the  lymph  flowing  from  the 
lymph  vessel^  into  the  venous  system.  The  walls  of  the  lymph 
vessels  are  always  thin ;  in  most  places  they  consist  merely  of 


ORGANS   OF  CIRCULATION. 


199 


epithelium  without  muscular  or  adventitial  envelopes,  and  at 
times,  as  in  the  frogs,  they  expand  into  large  subcutaneous 
lacunar  lymph  spaces,  or  similar  spaces  around  the  gonads  and 
in  the  mesenteries,  as  in  many  ichthyopsida.  The  system  is 
frequently  in  connection  with  the  coelom  by  means  of  openings 
(stomata)  in  the  peritoneal  membrane. 

The  distribution  of  these  vessels  varies  greatly  in  the  differ- 
ent groups ;  and  a  detailed  comparative  study  of  the  system  is 
still  a  desideratum,  while  its  development  is  largely  unknown. 
In  the  fishes  there  is  a  rich  plexus  of 
lymph  capillaries  beneath  the  skin  which 
extends  into  the  connective  tissue  be- 
tween the  muscles,  while  around  the 
heart  and  the  ventral  aorta  the  system 
is  richly  developed.  In  the  lower  ver- 
tebrates (amphibia,  reptiles,  and"  em- 
bryo birds)  pulsating  sacs  occur  in  the 
course  of  these  vessels,  —  the  so-called 
lymph  hearts.  These  are  usually  placed 
near  some  connection  between  the  lymph 
and  venous  systems,  as  near  the  pelvis 
and  the  caudal  vertebrae,  or  in  the  tho- 
racic cavity  dorsal  to  the  heart ;  but  oc- 
casionally lymph  hearts  occur  at  more 
distant  points.  For  instance,  in  the 
urodeles  a  series  of  these  occur  beneath 
the  lateral  line ;  none  are  known  in 
mammals. 

In  sauropsida  and  mammals  a  special 
trunk,  the  thoracic  duct,  is  developed  in 
connection  with  the  digestive  tract  which 
takes  the  lymph  from  the  hinder  extrem- 
ities, the  reproductive  and  excretory  or- 
gans as  well  as  the  alimentary  canal,  and  carries  it  forward,  pour- 
ing it,  in  the  sauropsida,  into  the  right  brachiocephalic  vein,  in 
the  mammals  into  the  left.1  In  birds  and  mammals  valves 

1  According  to  the  unpublished  studies  of  Dr.  F.  D.  Lambert,  a  paired  thoracic  duct  is 
developed  in  the  young  of  Amblystoma,  but  a  little  later  the  right  of  these  vessels  becomes 
obliterated. 


FIG.  209.  Urogenital 
system  of  tadpole  of  frog, 
after  Marshall  and  Bles. 
A,  radix  aortae;  £,  gills; 
F,  fore  foot ;  GF,  fat  body  ; 
GL,  glomus  of  head  kid- 
ney ;  GR,  genital  ridge  ; 
//,  heart ;  M,  mesone- 
phros ;  P,  pronephric  duct ; 
PR,  pronephros ;  U,  ureter. 


200     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


are  developed  in  the  larger  lymph  trunks,  preventing  any  back 
flow. 

In  connection  with  the  lymph  system  lymphoid  tissue  is- de- 
veloped, especially  around 
the  genital  organs  of  the 
ichthyopsida,  where,  as  in 
the  amphibia  and  reptiles, 
this  forms  the  prominent 
'fat  bodies.'  Aggrega- 
tions of  such  lymphoid  tis- 
sue give  rise  to  lymph 
glands,  which  are  variously 
distributed  through  the  ver- 
tebrate body.  Of  these 
the  most  prominent  is  the 
spleen,  the  cells  of  which 
are  said  to  arise,  in  the 
tadpole,  from  the  ento- 
derm.  It  usually  is  some- 
what close  to  the  stomach, 

and  in  Protopterus  (Fig.  40,  sp)  it  is  still  enclosed  in  the  gas- 
tric walls.  Among  other  lymph  glands  may  be  mentioned  the 
tonsils,  which  occur  at  the  beginning  of  the  pharynx  in  reptiles, 
birds,  and  mammals. 


FIG.  210.  Urogenital  organs  and  fat 
bodies  of  adult  frog.  C,  cloaca ;  F,  fat  body ; 
M,  mesonephros ;  P\  postcava ;  T,  testis ;  U, 
ureters. 


THE  SEGMENTATION  OF   THE   HEAD.  2OL 


THE  SEGMENTATION  OF  THE  HEAD. 

SINCE  in  the  vertebrates  the  region  of  the  body  behind  the 
head  is  made  up  of  segments  repeated  one  after  the  other 
(metamerism),  there  has  naturally  arisen  the  question,  Is  the 
head  itself  similarly  composed  of  somites  ?  If  so,  how  many 
of  these  somites  are  to  be  recognized  ?  Various  attempts  have 
been  made  to  solve  these  problems,  but  with  varying  results. 
Only  the  merest  outline  of  the  attempted  solutions  can  be  given 
here. 

In  the  trunk  and  tail  regions  the  parts  which  are  meta- 
merically  arranged  are  as  follows  :  myotomes,  spinal  nerves,  ver- 
tebrae, nephridial  tubes,  and  the  intersegmental  blood-vessels. 
Each  and  all  of  these  structures  have  been  employed  in  the. 
attempt  to  carry  the  segmentation  forward  into  the  head.  The 
existence  of  the  problem  was  first  recognized  by  Oken  (1807), 
who  attempted  its  solution  upon  a  vertebral  basis.  In  the 
mammalian  skull  he  recognized  three  vertebras,  the  centres  of 
which  were  represented  respectively  by  the  basioccipital,  sphe- 
noid and  ethmoid  bones,  while  the  neural  arches  were  formed 
by  ex-  and  supraoccipitals,  parietals,  and  frontals.  Later  stu-- 
dents  recognized  four  vertebrae  in  the  skull,  the  increase  being 
effected  by  recognizing  basi-  and  presphenoid  centres.1  In 
1869  Huxley  showed  that  this  theory  was  untenable,  and  that 
the  *  vertebrae '  of  the  skull  could  not  be  homologous  with  those 
of  the  trunk,  since  they  were,  in  part,  composed  of  membrane 
bone.  He  also  pointed  out  that  in  those  vertebrates  (elasmo- 
branchs)  where  one  would  naturally  expect  to  find  the  vertebrae- 
best  developed,  there  was  a  continuous  un segmented  brain  case. 
His  attempt  at  the  solution  of  the  problem  was  based  upon  the 
nerves  and  gill  clefts,  thus  transferring  the  question  from  the 

1  The  interested  student  will  find  the  extreme  development  of  this  '  vertebral  theory  ot 
the  skull '  in  the  first  volume  of  Owen's  '  Anatomy  of  the  Vertebrates.' 


2O2     MORPHOLOGY  OF  THE  ORGANS  OF  VERTEBRATES. 


T 


FIG.  211.  Diagram  of  the 
head  segments  in  a  selachian, 
after  Neal.  a,  anterior  so- 
mite ;  aa,  aortic  arch ;  al>, 
abducens  nerve ;  rfn,  dorsal 
nerves  ;  f,  facialis  nerve ;  g, 
glossopharyngeal  nerve ;  gc, 
gill  clefts;  A,  hypoglossal 
nerve;  if,  intestinal  branch 
of  vagus ;  Iv,  lateralis  branch 
of  vagus;  ;//,  mediolateral 
line;  n,  neuromeres ;  o,  otic 
vesicle  ;  oc,  oculomotor  nerve ; 
apt  ophthalmicus  profundus 
nerve  ;/>0,  post-trematic  nerve; 
fr,  pre-trematic  nerve;  s,  spi- 
racular  cleft ;  so,  mesodermic 
somites;  /,  trigeminal  nerve; 
v,  vagus  nerve  ;  7-AY,  neuro- 
meres; l-ii,  somites  of  van 
Wijhe;  1-7,  functional  gill 
clefts. 


vertebrae  to  the  neural  and  branchial 
segments.  With  these  as  a  basis  he 
recognized  nine  cranial  segments. 
Two  years  later  Gegenbaur,  using  the 
same  criteria,  also  concluded  that  there 
were  nine  segments  in  the  head,  al- 
though his  somites  and  those  of  Hux- 
ley do  not  agree  in  detail. 

Both  of  these  authors  recognized 
that  the  nerves  behind  the  ear  (IX- 
XII.)  were  like  the  spinal  nerves  in 
the  possession  of  dorsal  and  ventral 
roots,  and  that  the  ninth  divides  above 
the  first  gill  slit  into  pre-  and  post- 
trematic  branches  (p.  63).  The  tenth 
nerve,  however,  bears  similar  relations 
in  the  ordinary  sharks  to  four  gill 
clefts,  and  hence  is  a  compound  nerve. 
In  front  of  the  ear  the  facial  nerve 
divides  above  the  spiracular  cleft,  while 
the  trigeminal  nerve  splits  in  a  similar 
way  on  either  side  of  the  angle  of  the 
mouth.  This  last  circumstance  led 
Huxley  to  the  view  that  the  mouth 
has  arisen  from  the  coalescence  of  a 
pair  of  gill  slits,  a  view  which  has  re- 
ceived a  certain  amount  of  corrobora- 
tion  from  embryology.  This  left  a 
third  division  (ophthalmic)  of  the  fifth 
nerve  out  of  consideration  ;  this  was 
supposed  to  represent  another  seg- 
ment further  indicated  according  to 
Huxley's  view  by  the  orbito-nasal 
groove,  while  Gegenbaur  saw  traces  of 
it  in  a  pair  of  labial  cartilages.  Both 
recognized  an  additional  segment  in 
front  of  the  ophthalmic,  the  details  of 
which  are  not  necessary  here. 


THE  SEGMENTATION  OF   THE  HEAD.  203 

Balfour  introduced  another  element,  the  mesodermal  somites, 
into  the  discussion  ;  and  his  method,  developed  by  Marshall,1  and 
still  farther  by  van  Wijhe,  is  that  which  has  given  results  most 
often  quoted  in  connection  with  this  subject.  Van  Wijhe  con- 
sidered mesodermal  somites,  gill  clefts,  and  nerves,  and  tried  to 
utilize  the  purely  motor  nerves  (III,  IV,  VI,)  as  ventral  roots 
of  the  preauditory  nerves.  He  recognized  nine  mesodermal 
segments,  and  the  relations  of  these  to  the  segmental  nerves  is 
given  here  in  tabular  form. 

SOMITE.        DORSAL    NERVE    ROOT.  VENTRAL    NERVE    ROOT. 

1.  Ophthalmicus  profundus,  Oculomotor. 

2.  Trigeminal  less  op.  prof.,  Trochlearis. 

3.  )  (  Abducens. 

3     \    Acustico-faciahs, 

4.  )  I  None. 

5.  Glossopharyngeal,  None. 

Not  recognizable. 

a8us  JHypoglossal. 

Since  van  Wijhe  wrote,  others  have  tried  to  add  to  his 
structure,  and  some  have  claimed  to  recognize  eighteen  or  nine- 
teen of  these  head  somites.  It  is  pretty  certain  that  there  is  at 
least  one  somite  in  front  of  the  first  recognized  by  him  (Figs. 
121  and  122  a).  Others  have  taken  the  sense  organs  as  their 
b.isis,  including  in  this  not  only  ear  and  nose,  but  sensory  struc- 
tures developed  in  connection  with  the  gills,  and  considering  the 
ficialis  as  compound,  have  figured  out  eleven  head  segments 
(Heard).  Again,  the  early  condition  of  the  neural  tube  has 
shown  the  existence  of  nervous  segments  (neuromeres,  p.  49), 
eleven  in  number  in  the  head  region  (Orr,  Maclure,  Hoffmann) 
back  to  and  including  the  vagus.  Locy  has  claimed  that  the 
same  number  of  segments  can  be  recognized  in  the  medullary 
plate  of  elasmobranchs,  amphibia,  and  birds  before  it  is  infolded 
to  form  the  neural  tube,  but  his  conclusions  are  in  dispute. 

The  questions  asked  at  the  beginning  of  this  section  cannot 
as  yet  be  fully  and  finally  answered.  That  the  head  is  truly 

1  Balfour  recognized  eight  or  nine  somites ;  Marshall  nine  (eleven  in  notidanid 
sharks). 


204     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

segmented  can  hardly  be  doubted ;  but  this  region  of  the  verte- 
brates has  been  so  wonderfully  altered  and  specialized  that  the 
original  segments  have  been  greatly  changed,  and  in  some 
instances  may  have  disappeared.  The  postauditory  region  pre- 
sents the  simplest  condition  ;  the  tract  in  front  of  the  ear  is 
much  more  complex.  We  can  say  with  great  confidence  that 
there  are  many  more  than  the  three  somites  recognized  by 
Oken  ;  while  with  some  probability  we  can  say  that  the  number 
is  not  far  from  ten  or  eleven.  In  the  discussion  of  the  problem 
the  greatest  weight  should  be  given  to  the  positive  evidence  of 
the  myotomes,  since  it  is  probable  that  segmentation  originated 
in  the  mesoderm  ;  next  in  importance  are  the  cranial  nerves, 
while  less  weight  can  be  given  to  gill  clefts  and  their  modifi- 
cations, and  even  less  to  the  so-called  branchial  sense  organs. 


THE  EARLY  HISTORY  OF  THE   OVUM.  205 


THE  EARLY  HISTORY  OF  THE  OVUM. 

THE  formation  of  the  essential  parts  of  the  sexual  or  repro- 
ductive cells  from  the  germinal  epithelium  was  mentioned  upon 
a  preceding  page  (p.  125).  A  brief  account  of  their  subsequent 
history  follows.  Space  does  not  admit  of  any  extended  account 
of  the  details  of  the  phenomena  of  reduction  division,  matura- 
tion, impregnation,  etc.,  and  the  theories  based  upon  them  ;  for 
these,  reference  must  be  made  to  the  special  text-books  upon 
cytology  and  embryology. 

Both  eggs  and  spermatozoa,  as  they  leave  the  gonads,  are 
cells  specialized  for  the  perpetuation  of  the  species  ;  and  ulti- 
mate analysis  shows  that  as  they  leave  the  parent  tissue  these 
cells  contain  all  the  absolute  essentials  for  the  reproduction  of 
the  kind.  In  the  vertebrates,  however,  as  in  most  other  animals, 
these  essentials  are  variously  modified  in  shape  and  in  composi- 
tion by  the  addition  of  certain  secondary  features  which  demand 
attention. 

As  has  already  been  said,  the  ovum  is  a  specialized  cell, 
which  passes  into  an  ovarian  follicle,  and  receives  nourishment 
from  the  follicular  cells,  and  grows  larger  than  the  other  cells  of 
the  body.  At  last  it  escapes  from  the  ovary,  passes  into  the 
coelom,  and  thence  to  the  exterior  either  through  the  Mullerian 
ducts  (most  vertebrates),  through  the  pori  abdominales  (some 
teleosts),  or  by  means  of  special  structures  (many  teleosts).  In 
its  simplest  condition  an  ovum  is  directly  comparable  to  any 
other  cell  of  the  body,  consisting  merely  of  a  mass  of  proto- 
plasm with  a  specialized  portion,  the  nucleus,  near  the  centre. 
In  most  cases,  however,  it  receives  other  parts  of  a  secondary 
character,  either  from  the  ovarian  tissues,  or  from  the  walls  of 
the  ducts  through  which  it  passes. 

From  the  ovarian  tissue  the  egg  receives  a  varying  amount 
of  food  yolk  or  deutoplasm.  This  is  a  peculiar  substance  to  be 


206  MORPHOLOGY  OF  THE  ORGANS  OF  VERTEBRATES. 

used  later  as  food  by  the  growing  embryo.  It  appears  in  the 
shape  of  small  disks,  plates,  or  globules  embedded  in  the  proto- 
plasm. In  color  it  is  usually  white,  but  in  birds  (Fig.  212)  two 
kinds  of  yolk,  white  and  yellow,  are  arranged  in  a  complicated 
manner.  The  amount  and  distribution  of  this  deutoplasm  exer- 
cise an  important  influence  upon  the  early  phases  of  develop- 
ment, while  the  size  of  the  egg  is  directly  dependent  upon  the 
amount  of  this  substance. 

In  the  higher  mammals  the  deutoplasm  is  scanty  in  amount, 
and  is  regularly  distributed   throughout   the   protoplasm  (ale- 


.tf. 
& 


FIG.  212.  Diagram  of  hen's  egg,  from  Hertwig  after  Balfour.  ach,  air 
chamber;  bl,  blastoderm;  ch/,  chalaza;  ism,  inner  shell  membrane;  s,  shell;  sjn, 
outer  shell  membrane;  vt,  vitelline  membrane;  w,  white;  u>y,  white  yolk;  jc,  inner 
layer  of  white;  }'}',  yellow  yolk. 

cithal).  In  the  amphibia  and  in  Petromyzon  the  yolk  is  much 
more  abundant,  and  the  eggs  are  consequently  larger  in  size. 
It  is  still  distributed  throughout  the  whole  of  the  egg ;  but  a 
marked  polarity  is  visible,  one  pole  of  the  egg  containing  the 
bulk  of  the  protoplasm,  while  the  other  is  as  strongly  deutoplas- 
mic  (telolecithal).  In  selachians,  reptiles,  birds,  and  mono- 
tremes  this  polarity  is  still  more  marked ;  while  in  many  teleosts 
the  extreme  is  reached,  for  here  protoplasmic  and  deutoplasmic 


THE  EARLY  HISTORY  OF  THE   OVUM.  2O/ 

portions  are  sharply  distinct,  the  protoplasm  resting  as  a  small 
cap  upon  the  large  sphere  of  pure  food  yolk. 

The  egg  is  surrounded  by  primary  and  secondary  envelopes, 
the  former  arising  before  the  ovum  has  escaped  from  the  ovary, 
the  latter  from  the  ovarian  ducts.  In  the  vertebrates  the  pri- 
mary envelopes  are  at  most  three  in  number.  These  are,  from 
without  inwards,  (i),  a  vitelline  membrane,  structureless  in 
character ;  (2),  a  zona  radiata  (or  zona  pellucida)  traversed  by 
minute  pores ;  and  (3),  a  thin  and  delicate  inner  membrane. 
These  are  not  constant,  and  any  one  or  two  may  be  lacking 
in  a  given  egg.  In  some  cases  (teleosts  and  Petromyzon)  an 
opening  (micropyle)  exists,  through  which  the  spermatozoon 
obtains  entrance  to  the  egg. 

Of  the  secondary  envelopes  one  of  the  simplest  conditions 
is  found  in  Petromyzon,  where  the  outer  surface  of  the  egg  is 
covered  with  a  thin  layer  of  adhesive  mucus,  which  serves  to 
fasten  the  egg  to  stones,  etc.  In  the  myxinoids  the  egg  envel- 
ope is  more  horny,  and  is  provided  at  either  end  with  anchoring 
hooks.  The  descriptions  would  also  imply  that  at  the  time  of 
laying  there  was  an  outer  sheathing  capsule.1  In  the  amphibia 
the  eggs  receive  a  coating  in  their  passage  down  the  oviduct 
which  swells  into  a  jelly  when  in  contact  with  water.  In  the 
elasmobranchs  the  eggs  are  enclosed  in  a  horny  capsule,  usually 
quadrangular  in  outline,  while  in  the  reptiles  and  monotremes 
the  oviducts  secrete  around  each  egg  a  calcareous  shell. 

The  birds  present  the  most  complicated  condition.  Here 
the  eggs,  after  they  have  entered  the  oviduct,  receive  first  a 
layer  of  albumen  (the  '  white '),  a  portion  of  which,  firmer  than 
the  rest,  is  twisted  into  a  spiral  chalaza  at  either  end.  Outside 
of  this  there  is  next  deposited  a  double  shell  membrane,  and 
then,  by  the  next  division  of  the  duct,  the  calcareous  shell 
(Fig.  212). 

The  spermatozoa  arise  in  the  canaliculi  seminiferi  of  the 
testes  (p.  126),  but  they  present  many  differences  from  the  eggs. 
These  in  merest  outline  are  as  follows  :  In  every  cell  of  the 

1  No  one  has  yet  described  the  origin  of  these  envelopes  of  the  cyclostome  egg  ;  it  may 
be  that  they  are  ovarian  in  origin,  a  view  which  seems  the  more  probable  from  the  absence 
of  oviducts. 


208     MORPHOLOGY  OF  THE   ORGANS   OF   VERTEBRATES. 


body  in  any  given  species  the  nucleus  contains  a  fixed  and 
•definite  number  of  bodies,  the  chromosomes,  so  called  because 
they  are  readily  colored  by  the  various  microscopical  stains. 
-Attach  division  of  a  cell  these  chromosomes  are  divided  so  that 
each  daughter-cell  contains  exactly  as 
many  as  did  the  mother-cell.  Each  egg 
before  it  leaves  the  ovary  contains  these 
chromosomes,  and  the  number  in  each 
egg  corresponds  exactly  with  the  number 
in  any  other  part  of  the  body  of  the 
mother.  In  the  formation  of  the  sper- 
matozoa, however,  there  is  a  peculiar 
cell  division,  —  a  so-called  reduction  di- 
vision,—  the  results  of  which  are  that 
each  resulting  spermatozoon  contains  just 
half  the  number  of  chromosomes  normal 
to  the  species. 

The  spermatozoa  also  differ  from  the 
eggs  in  their  appearance.  The  egg  is 
passive,  and  it  contains  the  nourishment 
and  material  from  which  the  young  is  to 
be  developed.  The  spermatozoon,  on  the 
other  hand,  must  be  active;  for  it  must 
seek  out  and  unite  with  the  egg  in  order 
that  the  latter  may  develop.  To  this  end 
it  is  made  as  small  as  possible.  Deuto- 
plasm  is  entirely  absent,  and  the  extra- 
nuclear  protoplasm  is  reduced  to  the 
smallest  amount.  The  chromosomes  are 
compacted  into  a  small  body,  the  so- 
called  head,  while  the  protoplasm  is 
largely  developed  into  a  'tail,'  consisting 
of  an  axial  filament  and  a  lateral  mem- 
brane, by  means  of  which  the  spermatozoon  is  able  to  swim. 

Impregnation  consists  of  the  union  of  the  egg  and  the 
spermatozoon,  and  there  is  abundant  evidence  to  show  that  a 
single  spermatozoon  is  sufficient  to  impregnate  a  single  egg. 
This  impregnation  may  take  place  either  outside  or  inside  of 


FIG.  213.  A,  human 
spermatozoon  front  and 
side  view,  after  Retzius  ; 
B,  diagram  of  vertebrate 
spermatozoon,  modified 
from  Bohm  and  Davidoff. 
&/;  axial  filament ;  e,  end 
piece  of  Retzius;  k,  head  ; 
j/i,  middle  piece  ;  mb,  un- 
dulating membrane;  mft 
marginal  filament ;  sf,  sec- 
ondary filament;  /,  tail. 


THE   EARLY  HISTORY  OF  THE   OVUM.  209 

the  body  of  the  mother,  the  latter  being  the  prevailing  method, 
external  impregnation  occurring  only  in  the  cyclostomes  and  in 
most  teleosts  and  amphibia.  Occasionally,  as  in  some  urodeles, 
the  spermatozoa  are  deposited  in  bunches  (spermatophores) , 
which  are  taken  into  the  cloacal  opening,  effecting  internal 
impregnation  ;  or,  as  in  most  elasmobranchs,  the  ventral  fins  of 
the  males  are  modified  into  copulatory  and  intromittent  organs 
(claspers).  In  the  birds  the  transmission  of  the  sperm  to  the 
female  is  effected  by  an  apposition  of  cloacal  openings,  although 
in  a  few  birds  a  copulatory  organ,  the  penis,  is  developed.  In 
the  reptiles  this  structure  acquires  a  greater  development,  and 
reaches  its  extreme  in  the  mammals. 

After  the  spermatozoon  has  penetrated  into  the  egg,  there 
occurs,  in  all  eggs  accurately  studied,  certain  phenomena  which 
constitute  the  process  known  as  maturation.  These  chiefly 
concern  the  nucleus,  and  are  as  follows :  The  nucleus  ap- 
proaches the  surface  of  the  egg  and  undergoes  a  normal  divis- 
ion, one  of  the  resulting  halves,  together  with  a  small  amount 
of  protoplasm,  being  cast  out  of  the  egg  as  the  first  polar 
globule.  The  nucleus  now  divides  again  ;  but  this  division  is  a 
reduction  division,  half  of  the  chromosomes  being  cast  out  in 
a  second  polar  globule,  while  half  sink  back  into  the  egg,  which 
now  contains  just  as  many  chromosomes  as  does  the  sperma- 
tozoon. These  chromosomes  now  unite  with  those  from  the 
head  of  the  spermatozoon,  forming  the  new  nucleus  of  the  egg 
(the  segmentation  nucleus),  which  thus  again  contains  the  num- 
ber of  chromosomes  characteristic  of  the  species.  Not  until 
this  process  is  complete  is  the  egg  really  impregnated  and  ready 
for  segmentation. 

The  character  of  the  segmentation  varies  according,  among 
other  things,  to  the  amount  and  distribution  of  the  food  yolk 
in  the  egg.  This  substance  is  inert,  and  its  presence  interferes 
with  the  living  and  active  protoplasm.  Were  no  deutoplasm 
present,  the  egg  would  divide  in  a  regular  and  equal  manner, 
and  the  resulting  cells  would  be  equal  in  size  ;  and  the  same 
would  be  true,  other  things  being  equal,  were  the  deutoplasm 
small  in  amount  and  evenly  distributed  through  the  protoplasm. 
Such  eggs  do  occur  in  the  non-vertebrate  groups,  but  none  are 


210     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 


known  among  the  vertebrates.      The  simplest  conditions  pre- 
sented  by  these    forms  may   be    illustrated   by  the  amphibia, 

the  outlines  which 
follow  being  to  a 
measure  true  of  Petro- 
myzon  and  some  gan- 
oids. 

In  the  amphibia 
the  first  two  planes 
of  division  may  be 
compared  to  two  me- 


FIG.  214      Two  stages  in  the  segmentation  of  the 
egg  of  Amblystoma;  i  to  5  the   successive  planes  of 

segmentation. 


Q£ 


right  angles  to  each 
other.      These  begin 

to  cut  through  at  the  protoplasmic  (darker)  pole  of  the  egg, 
and  gradually  extend  to  the  other.  The  third  plane  is  at  right 
angles  to  these,  but  nearer  the  protoplasmic  than  to  the  deu- 
toplasmic  pole.  The  result  is  that  the  eight  resulting  cells 
are  unequal  in  size,  four  being  small  and  four  much  larger, 
This  disparity  in  size  is  continued  in  the  following  divisions,  and 
it  also  affects  the  position  of  the  internal  segmentation  cavity 
(p.  5),  which,  instead  of  being  central, 
is  pushed  toward  the  protoplasmic 
pole  (Fig.  215).  In  the  amphibia, 
then,  the  whole  egg  divides  into  cells. 
Such  eggs  are  called  holoblastic. 

In  elasmobranchs,  reptiles,  and 
birds,  where  the  deutoplasm  is  much 
more  abundant,  and  the  polar  differ- 
entiation of  the  egg  is  more  marked, 
the  planes  of  segmentation  do  not  FIG.  215.  Early  stage  of  the 

CUt   through  the  entire  egg,   but    are      segmentation  of   the  egg  of  Am- 


blystoma, in  section,  showing  the 
excentric  position  of  the  segmen- 
tation cavity,  s. 


confined  to  what  is  called  the  ger- 
minal area  at  the  protoplasmic  pole. 
Here  occur  meridional  and  circular 
planes  of  division,  so  that  the  germinal  area  is  converted  into 
cells,  while  the  bulk  of  the  egg  remains  unsegmented.  In 
these  meroblastic  eggs  the  segmentation  cavity  is  still  farther 


THE  EARLY  HISTORY  OF   THE   OVUM. 


211 


displaced  from  the  centre,  so  that  it  comes  to  lie  immediately 
beneath  the  protoplasmic  pole.  The  layer  of  cells  formed  by 
this  segmentation  is 

SET 


known  as  the  blasto- 
derm. In  the  teleosts 
where  protoplasm  and 
deutoplasm  are  sharply 
distinct,  the  protoplas- 
mic portions  alone  are 
segmented,  the  food 
yolk  remaining  undi- 
vided, and  the  segmen- 
tation cavity  here  lies 
between  the  blastoderm  and  the  deutoplasmic  mass. 

The  little  that  is  known  of  the  development  of  the  eggs  of 
the  monotremes  shows  that  in  their  features  of  segmentation 
they  are  closely  like  the  reptiles  and  birds,  —  they  are  mero- 
blastic  ;  but  the  other  mammals  present  several  peculiarities  in 
their  segmentation,  which  can  best  be  considered  after  a  descrip- 
tion of  the  process  of  gastrulation  in  other  vertebrates. 

In  the  amphibia  and  similar  forms  the  segmentation  cavity 
is  so  small  that  it  would  be  impossible  for  the  larger  yolk-loaded 


FlG.  216.  Early  segmentation  of  hen's  egg, 
after  Duval.  e,  ectoderm  ;  /,  lower  layer  cells ; 
s,  segmentation  cavity ;  w,  white  yolk  ;  y,  yel- 
low yolk.  Only  a  small  part  of  the  egg  shown; 
compare  Fig.  212. 


FlG.  217.     Diagram  of  the  process   of  gastrulation  in    Amblystoma ;  the  cells 
to  be  invaginated  as  entoderm  shaded,     a  and  b,  front  and  side  views  of  the  beginning 
of  the  invagination,  the  primitive  groove  beginning  in  b',  c,  a  later  stage  with  longer 
primitive  groove ;  in  d,  the  process  is  nearly  complete,  a  small  patch  of  entode/tw 
being  seen  behind  (yolk  plug  in  the  '  anus  of  Rusconi  '). 

cells  to  become  invaginated  in  the  typical  manner  (p.  5),  and 
so  a  modification  of  the  process,  not  easily  described,  takes  place, 
the  result  however  being  the  production  of  the  two-layered  gas- 
trula.  In  a  few  words  the  process  may  be  outlined  as  follows  : 
Since  the  large  cells  cannot  be  pushed  inside  the  smaller  ones, 


212     MORPHOLOGY  OF   THE   ORGANS   OF   VERTEBRATES. 

the  latter  grow  down  over  the  others  at  that  side  of  the  egg 
which  is  later  to  form  the  hinder  end  of  the  embryo.  This 
downward  growth  results  in  the  infolding  of  a  portion  of  the 
cells  as  a  sort  of  pocket,  the  cells  on  the  upper  surface  of  the 
pocket  being  small,  those  on  the  lower  side  the  larger  yolk-laden 
cells.  This  pocket  is  the  archenteron  ;  its  walls  are  entoderm, 
and  its  opening  is  the  blastopore.  As  the  process  continues,  the 
blastoporal  lips  of  either  side  come  to  meet  in  the  median  line, 
producing  the  primitive  streak  and  groove  already  described. 

In  the  elasmobranchs  the  process  of  gastrulation  is  still  far- 
ther modified  by  the  large  amount  of  unsegmented  yolk.     Here 


A  B 

FIG.  218.  Diagram  of  two  stages  in  the  formation  of  the  embryo  in  an  elasmo- 
branch  egg  ;  the  inflected  rim  of  the  blastoderm  divided  up  into  segments  so  as  to 
illustrate  the  formation  of  the  embryo  by  concrescence. 

the  blastoderm  forms  a  small  circular  patch  of  cells  resting  upon 
the  yolk.  At  the  morphologically  hinder  portion  of  this  blasto- 
derm the  cells  begin  to  turn  in  between  the  rest  and  the  yolk, 
thus  differentiating  ectoderm  and  entoderm.  As  the  blastoderm 
increases  in  size  by  continual  cell  division  plus  additions  from 
the  yolk,  this  infolded  rim  grows  together,  its  halves  swinging 
in  toward  the  middle  line  so  that  the  grooves  of  either  side  unite 
to  form  a  tube,  —  the  archenteron,  —  the  floor  of  which  is  formed 
by  the  yolk.  The  rim  of  the  inflected  tissue  must  be  regarded 
as  the  lips  of  the  blastopore,  and  as  these  lips  unite  in  the  median 
line  they  give  rise  to  the  primitive  streak. 


THE  EARLY  HISTORY  OF   THE   OVUM. 

In  birds  the  first  phenomenon  of  gastrulation  is  the  forma- 
tion of  a  crescentic  or  sickle-shaped  groove  at  the  margin  of  the 
blastoderm,  the  anterior  margin  of  which  is  directly  comparable 
to  the  rim  of  inflection  in  the  elasmobranch.  The  edges  of  the 
right  and  left  halves  of  this  groove  coalesce  as  in  the  sharks, 
and  then  the  blastoderm  grows  backward  beyond  the  primitive 
streak  thus  formed,  so  that  the  streak  comes  to  lie  like  an  island 
in  the  centre  of  the  blastoderm.  In  the  reptiles  much  the  same 
conditions  occur  as  in  the  birds,  except  that  the  blastopore  is 
placed  within  rather  than  at  the  edge  of  the  blastoderm. 

A  feature  to  be  noticed  in  all  the  foregoing  types  is  that  in 
each  case  the  embryo  arisesi rom  right  and  left  portions,  which  at 
first  may  be  widely  separate,  and  which 
meet  and  fuse  in  the  middle  line.    This  A..^^p^;?^, 

phenomenon   of    concrescence   consists          °"^r:'- :  ;;;Vx       Vf8&. 
in  the  formation  of  the  dorsal  portions          J£: 
of  the   embryo,  and   all   of  the   struc-      p?§Mw>:- 
tures   there  developed  —  nervous  sys-      g^;%-- 
tern,  myotomes,  sclerotomes,   vascular      c  J::ffi: 
system,     and     ccelomic     structures  — 
from  the  union  of  the  blastoporal  lips.1 
This  process   is  illustrated  in  our  fig- 
ures  (Fig.  218)  where  the  successive         FIG.  219.     Early    blasto- 
portions    of    the    germinal    ring    (i.e.,    derm  of  hen's  egg>  after  K5J- 
edges  of   the    blastopore)  are    shown    liker'    '' crescent ;  *' primitive 

0  .  '  groove ;  o,  area  opaca ;  /,  area 

uniting  to   form    the    axial   portion  of    peiiucida. 
the  embryo. 

In  the  placental  mammals  the  eggs  are  very  small  and  the 
amount  of  deutoplasm  is  small,  consequently  the  eggs  are  holo- 
blastic.  In  their  segmentation  and  in  the  method  of  formation  of 
the  germ  layers  these  eggs  present  many  peculiarities,  which  are 
usually  explained  upon  the  hypothesis  that  the  mammals  have 
descended  from  animals  with  large-yolked  eggs,  and  that  the 
features  in  which  they  differ  from  the  other  vertebrates  as  well 
as  from  other  animals  with  holoblastic  eggs  are  to  be  attributed 

1  Several  authors  (Sedgwick,  Kastschenko,  Morgan,  and  others)  have  criticised  this 
theory  in  one  way  or  another,  but  the  actual  facts  of  development  seem  to  negative  their 
arguments. 


214     MORPHOLOGY  OF   THE   ORGANS  OF   VERTEBRATES. 


to  loss  of  yolk  and  consequent  modification  of  processes.  From 
almost  the  first  the  segmentation  is  irregular ;  and  there  results, 
exactly  how  is  not  known,  a  solid  sphere  consisting  of  an  outer 
layer  of  hyaline  cells  surrounding  a  mass  of  more  granular  cells, 
one  of  which  reaches  to  the  exterior  through  a  gap,  closed 
later,  between  the  cells  of  the  outer  layer.  Now  the  solid  mass 

expands  into  a  hollow  sphere,  the 
blastodermic  vesicle,  the  outer 
layer  of  cells  becoming  greatly 
flattened,  the  inner  adhering  to 
one  side  of  them  in  a  small  len- 
ticular mass,  the  cavity  of  the 
vesicle  being  filled  with  fluid. 
The  lenticular  mass  becomes 
three  layered,  increases  in  extent, 
and  gradually  extends  around 
under  the  outer  layer  so  that  the 
whole  vesicle  is  eventually  two 
layered  throughout.  The  cen- 
tral portion  of  the  lenticular 
mass  remains  thicker  than  the  rest,  and  in  this  place  the  em- 
bryo arises,  a  primitive  streak  being  formed,  but  nothing  that 
can  with  certainty  be  called  a  blastopore  appears. 

So  far  there  is  little  dispute  as  to  the  facts,  but  as  to  their 
interpretation  the  views  are  various,  some  regarding  the  outer 
layer  as  ectoderm,  others  as  entoderm ;  while  the  inner  cell  mass 
is  regarded  by  some  as  purely  entodermal,  by  others  as  giving 
rise  to  both  ectoderm  and  entoderm.  For  details  reference 
must  be  made  to  special  works  on  vertebrate  embryology. 


FlG.  220.  Diagram  of  mammalian 
blastodermic  vesicle.  *',  inner  cell 
mass. 


THE   ORIGIN  OF  THE    VERTEBRATES.  21$ 


THE    ORIGIN    OF   THE   VERTEBRATES. 

THE  question  as  to  the  ancestors  of  the  vertebrates  is  one  of 
the  most  vexed  problems  of  zoology.  It  has  seemed  at  times 
as  if  the  solution  were  near  at  hand.  The  recognition  of  chor- 
date  affinities  in  the  tunicates,  and,  later,  in  Balanoglossus,  at 
the  time  when  these  were  regarded  as  invertebrates,  raised 
hopes  that  were  disappointed  when  it  was  found  that  these 
forms  were  chordates,  and  that  only  superficial  resemblances 
had  caused  their  association  with  the  non-vertebrate  groups.  It 
would  seem  that  to-day  we  are  not  much  nearer  the  answer  to 
the  question  than  we  were  when  the  theory  of  evolution  was  new. 

Apparently  the  problem  must  be  solved,  if  solved  it  ever 
be,  upon  the  basis  of  comparative  anatomy  and  embryology. 
Paleontology  has  never  thrown  the  slightest  light  upon  the 
matter,  and  it  seems  as  if  it  never  could,  because  it  is  more 
than  probable  that  the  ancestral  chordate  was  a  soft-bodied 
animal  of  small  size,  incapable  of  leaving  any  definite  impress 
in  the  rocks. 

The  three  most  important  characteristics  of  the  vertebrates, 
and  of  all  chordates,  are  the  presence  of  gill  slits,  the  existence 
of  a  notochord,  and  the  occurrence  of  a  central  nervous  system 
placed  entirely  upon  one  side  of  the  alimentary  canal.  These 
features  are  found  in  no  invertebrate,  and  we  can  only  speculate 
upon  the  way  in  which  they  have  arisen  ;  for  it  is  one  of  the 
canons  of  evolution  that  no  organ  arises  de  novo,  but  only  by 
modification  of  some  pre-existing  structure. 

At  present  the  greater  weight  of  evidence,  such  as  it  is, 
points  toward  an  annelid  ancestry.  Annelids  and  vertebrates 
agree  in  the  possession  of  metamerism,  and  the  homologies  of 
the  metameric  structures  can  be  traced  with  some  detail.  Mus- 
cular system,  ccelomic  pouches,  and  nephridia  agree  in  their 
general  features,  while  the  fact  that  the  nephridial  ducts  in  both 


2l6     MORPHOLOGY  OF  THE   ORGANS  OF   VERTEBRATES. 

serve  to  carry  away  the  sexual  cells  is  also  suggestive.  These 
ducts,  however,  afford  some  difficulties,  as  it  is  not  easy  to  see 
how  the  continuous  pronephric  duct  of  the  vertebrates  could 
have  arisen  from  the  separate  ducts  of  the  annelid.  Again,  the 
ventral  nervous  chain  of  the  annelid  can  be  closely  compared 
with  the  spinal  cord  of  the  vertebrates,  the  comparison  includ- 
ing the  dorsal  roots  of  the  nerves  with  the  spinal  ganglia.  The 
same  is  true  of  the  similarities  existing  between  the  transverse 
blood-vessels  of  the  annelids  and  the  aortic  arches  and  inter- 
costal vessels  of  the  vertebrates. 

The  most  plausible,  hypothesis  by  which  to  homologize  the 
anterior  portions  of  the  nervous  system  is  that  which  regards 
the  infundibulum  and  the  nervous  portion  of  the  hypophysis  as 
representing  the  invertebrate  mouth,  while  the  vertebrate  mouth 
may  have  arisen  by  the  coalescence  of  a  pair  of  gill  slits.  In 
this  case  the  '  brain '  of  the  annelid  would  be  represented  by 
the  vertebrate  fore  brain.  In  certain  annelids  there  exists  a 
subintestinal  tube  of  entodermal  origin  which  has  been  doubt- 
fully compared  with  the  notochord,  but  as  yet  no  structures  are 
known  in  the  annelids  which  can  be  homologized  with  the  gill 
slits. 

Other  but  less  widely  accepted  views  of  the  ancestry  of  the 
vertebrates  are  those  which  would  derive  the  group  from  some 
arthropod  not  far  from  the  limuloids,  or  from  the  nemertean 
worms.  It  must  however  be  kept  in  mind  that  the  great- 
est resemblances  between  vertebrates  and  annelids  are  directly 
or  indirectly  the  result  of  metamerism  ;  and  that  it  is  possible 
that  this  vegetative  repetition  of  parts  may  have  arisen  inde- 
pendently in  the  chordate  phylum,  and  that  the  similarities 
note:!  above  may  be  expressions  of  convergent  evolution,  and 
that  the  chordates  may  have  descended  from  non-segmented 
ancestors.  This  view  receives  some  support  from  the  fact  that 
metamerism  also  occurs  in  the  echinoderms,  where  it  could  not 
have  been  inherited  from  either  annelids  or  vertebrates. 

In  the  following  pages  are  numerous  references  to  the  lines 
of  descent  of  the  various  groups  of  vertebrates.  The  adjacent 
diagram  illustrates  some  of  these.  Concerning  some  points 
there  are  differences  of  opinion.  Thus  the  dipnoans  are  fre- 


THE   ORIGIN  OF  THE    VERTEBRATES. 


217 


quently  regarded  as  the  ancestors  of  the  amphibia,  a  view 
which  receives  its  chief  support  from  the  existence  of  lungs 
in  both  forms.  In  their  skeleton  and  in  other  features  the 
two  groups  seem  widely  remote.  Again,  in  recent  years  the 


MAMMALS 


BIRDS 
REPTILES 


TELEOSTS 


GANOIDS 


3OCEPHALS 
CROSSOPTERYGI1 


HOLOCEPHAH 

ELASMOBRANCHS 


?  CYCLOSTOMES 
PRIM.  ELAS 


?  OSTRACODERMS 
dOBRANCHS 


FlG.  221.     Lines  of  descent  of  the  different  groups  of  vertebrates. 

tendency  has  been  to  regard  the  mammals  as  descendants  from 
theromorphous  reptiles,  a  view  which  receives  its  chief  evidence 
from  paleontology.  More  lately  still  the  tendency  is  to  revert 
to  the  older  view  of  an  amphibian  ancestry. 


• 


PART    II. 
SYSTEMATIC   ZOOLOGY. 


SUB-PHYLUM    VERTEBRATA. 

METAMERIC  metazoan  animals  with  a  complete  alimentary 
canal,  the  anterior  portion  of  which,  at  least  in  embryonic 
stages,  is  provided  with  gill  slits.  The  central  nervous  system, 
consisting  of  brain  and  spinal  cord,  is  hollow,  and  is  situated 
•entirely  on  one  side  of  the  alimentary  tract.  Between  alimen- 
tary canal  and  central  nervous  system  is  a  skeletal  axis  —  the 


YiG.  222.  Anatomy  of  a  vertebrate,  based  on  Amblystoma.  a,  anus;  £,  brain; 
/%,  heart ;  /,  lung  ;  //',  liver  ;  o ,  ovary  ;  od,  oviduct ;  <?/,  ostium  tubse  ;  /,  pelvis ;  s, 
stomach ;  j/,  sternum ;  /,  tongue ;  /;-,  trachea ;  u,  urinary  bladder. 

notochord  —  of  entodermal  origin,  which  may  persist  throughout 
life.  Around  this  notochord  is  developed  a  segmented  skeleton, 
•consisting  of  skull  and  vertebral  column.  The  heart  is  on  the 
.abneural  side  of  the  alimentary  canal,  and  consists  of  at  least 
two  chambers.  It  connects  with  a  dorsal  aorta  by  arterial 
;arches,  which,  however,  may  become  greatly  reduced  and  modi- 
fied in  the  adult.  The  blood  contains  red  corpuscles  in  addition 
"to  leucocytes.  The  reproduction  is  entirely  sexual.  The  verte- 
brates are  free  throughout  their  entire  existence. 

218 


C  YCL  OS  TOMES.  2 1 Q 

In  dealing  with  the  classification  of  the  vertebrates  many  dif- 
ferent ideas  exist,  not  only  with  regard  to  the  interrelationships 
of  the  various  groups,  but  with  their  co-ordination  as  well.  This 
is  due  to  several  causes.  Among  them  may  be  mentioned  the 
fact  that  only  in  exceptional  cases  among  the  fossil  vertebrates 
are  other  structures  than  the  skeleton  •  preserved,  and  for  this 
reason  our  classifications  have  of  necessity  been  too  largely 
based  upon  osteological  characters.  Again,  there  is  a  great  dif- 
ference in  the  numbers  of  species  in  the  different  groups ;  thus 
the  cyclostomes,  one  of  the  two  great  divisions  of  vertebrates, 
contain  less  than  a  score  of  species,  while  of  birds  about 
twelve  thousand  '  species  '  are  known.  As  a  result,  the  group 
of  aves  has  been  subdivided  to  an  extent  unknown  in  other 
classes.  Divisions  which  elsewhere  would  be  regarded  as  fami- 
lies are  here  raised  to  ordinal  rank,  and  other  subdivisions  cor- 
respondingly magnified.  In  the  following  pages  it  has  been 
the  attempt  to  preserve  a  proper  co-ordination  of  groups,  —  to 
maintain  a  classificatory  perspective. 

The  vertebrate  phylum  may  be  divided  into  cyclostomes  and 
gnathostomes. 

Series    I.     Cyclostomata    (Agnatha). 

Eel-like  vertebrates  without  paired  appendages  ;  mouth  suc- 
torial, jaws  lacking;  olfactory  organ  single  and  median;  optic 
nerves  going  to  the  eyes  of  the  same  side;  gills  6-14  pairs  in 
saccular  pouches.     The  cyclostomes  include  but  a  single  class, 
—  the  Marsipobranchii. 

CLASS  I.     MARSIPOBRANCHII   (MYZONTES). 

The  marsipobranchs  are  undoubtedly  the  lowest  vertebrates  ; 
but  there  is  yet  a  question  as  to  how  far  their  simple  structure 
is  the  result  of  a  primitive  condition,  and  how  far  it  has  been 
caused  by  degeneration.  The  body  is  eel-like  ;  and  all  traces  of 
paired  fins  are  absent,  unless,  as  Dohrn  suggests,  two  slight  folds 
near  the  vent  are  the  remnants  of  ventral  fins.  A  median  fin 
occurs,  which  may  be  continuous,  or  may  be  differentiated  into 


220 


CLASSIFICATION    OF    VERTEBRATES. 


dorsals  and  caudal.     The  skin  is  without  scales,  but  is  rich  in 
mucus-secreting  cells,  and  in  the  myxinoids  contains  also  nu- 
merous pockets   of    so-called 
thread    cells,    these    pockets 
extending    into   the  underly- 
ing  muscles.     These   thread 
cells    have    their    protoplasm 
FIG.  223.'  Thread  cells  of  Bdellostoma,     converted  into  a  long  thread, 

one    intact,    the   other    'exploded,'    after       and      when      these      are      dlS- 

Ayers*  charged,  the  threads  become 

unwound  ;  and  these  and  the  mucus  are  so  abundant  that  one 
of  these  animals  will  convert  a  bucket  of  water  into  a  thick  jelly. 

The  mouth  is  at  the  bot- 
tom of  a  more  or  less  circular 
suctorial  funnel,  the  inside  of 
which,  like  the  tip  of  the 
mobile  tongue,  is  armed  with 
horny,  cuticular  teeth,  which 
aid  the  suckers  in  anchoring 
these  animals  to  the  fish  on 
which  they  feed,  and  also  serve 
to  rasp  the  flesh.  The  alimen- 
tary canal  is  straight  ;  a  cir- 
cular fold,  the  velum,  occurs 
inside  the  mouth  ;  no  cloaca  is 
present.  The  brain  has  well- 
developed  cerebral  lobes  which 
may  be  solid  or  hollow,  but 
the  cerebellum  is  very  small. 
The  nasal  organ  is  median,  and 
is  placed  at  the  posterior  side 


of  the  hypophysial  duct,  the 
opening  to  which  thus  serves 
as  the  nostril,  opening  either 
at  the  tip  of  the  snout  (myxi- 
noids) or  upon  the  top  of  the 
head  (petromyzontes).  The 


FlG.  224.  Brain  and  nasal  organ  of 
Petromyzon.  In  front  is  (darker)  the 
nasal  canal,  behind  which  are  the  plaits  of 
the  nasal  membrane.  On  either  side  of 
the  twixt  brain  are  bits  of  the  cartilage 
of  the  chondrocranium,  and  farther  back 
the  otic  capsules. 


deeper  end  of  the  hypophysis  expands   into  a  large  sac,  which 


CYCLOSTOMES.  221 

in  the  myxinoids,  opens  into  the  mouth.  The  ears  are  remark- 
able in  the  absence  of  the  horizontal  (external)  semicircular 
canal,  while  in  the  myxinoids  but  a  single  canal  is  present, 
which,  as  it  bears  an  ampulla  at  either  end,  may  be  regarded 
as  representing  the  anterior  and  posterior  canals  of  the  normal 
ear  (p.  71). 

The  vertebral  column  consists  of  a  large  persistent  noto- 
chord  surrounded  by  a  fibrous  sheath  and  a  membranous  neural 
tube,  in  which  {Petromyzoti)  cartilaginous  neural  arches  occur. 
The  cranium  is  cartilaginous,  but  is  more  or  less  incomplete 
above,  and  roofed  by  membrane.  Labial  cartilages  and  carti- 


G 

FlG.  225.  Cranium  and  branchial  basket  of  Pctromyzon,  after  W.  K.  Parker. 
E,  otic  capsule ;  B,  branchial  basket ;  G,  gill  clefts ;  N,  nasal  capsule ;  NTt 
notochord. 

lages  for  the  tongue  occur ;  but  all  traces  of  jaws  —  pterygoquad- 
rates,  Meckel's  cartilage  —  are  lacking.  The  branchial  region 
in  the  petromyzontes  is  supported  by  a  complicated  cartilagi- 
nous framework,  the  branchial  basket ;  but  it  is  as  yet  impossible 
to  homologize  this  with  the  visceral  arches  of  the  higher  verte- 
brates. In  the  myxinoids  the  basket  is  rudimentary. 

The  gill  slits  are  tubular,  and  the  folded  gills  are  borne  on 
the  walls  of  pouch-like  enlargements  of  these  tubes.  The  heart 
lacks  a  conus  arteriosus,  and  no  renal  portal  circulation  occurs. 
The  excretory  organs  are  elongate  bands.  In  the  lampreys  and 
Myxine  the  pronephros  is  lost  in  the  adult  ;  but  in  Bdellostoma 
it  retains  its  function  throughout  life,  its  nephrostomes  open- 
ing into  the  pericardium.1  The  gonads  are  unpaired,  and  in  the 
myxinoids  are  protandric  hermaphrodite  in  character  (i.e.,  the 
animal  is  at  one  time  male  at  another  female),  the  anterior  por- 

1  According  to  Price,  the  whole  excretory  organs  of  Bdellostoma  are  pronephric  in 
character. 


222 


CLASSIFICATION    OF    VERTEBRATES. 


tion  being  ovary,  the  posterior  testis.  The  sexual  products  are 
discharged  into  the  body  cavity,  from  which  they  escape  to  the 
exterior  through  genital  pores  which  open  into  the  hinder  end 
of  the  urinary  (pronephic)  ducts. 

The  eggs  of  the  myxinoids  are  large,  and  each  bears  at  either 
end  a  crown  of  long-stalked  anchoring-hooks.  Almost  nothing 
is  known  of  the  development.  The  lampreys 
have  much  smaller  eggs,  the  early  development 
of  which  shows  striking  similarities  to  the  con- 
ditions found  in  the  amphibia.  The  eggs  un- 
dergo a  total  but  unequal  segmentation,  with 
small  cells  (micromeres)  at  one  pole,  and  larger 
yolk-laden  cells  (macromeres)  at  the  other. 
Gastrulation  is  effected  in  a  modified  manner 
by  a  growth  of  the  micromeres  over  the  mac- 
romeres, and  the  blastopore  (or  rather  its  pos- 
terior end)  persists  as  the  anus  of  the  adult. 
In  the  development  of  the  nervous  system,  in- 
stead of  the  typical  inrolling  of  a  medullary 
plate  there  is  formed  a  solid  cord  or  keel  of  cells  along  the 
middle  line  of  the  back,  in  which  later,  by  splitting,  a  cavity 
appears.  At  the  extremity  of  the  head  appears  an  inpushing, 
from  the  walls  of  which  both  olfactory  organ  and  hypophysis 
are  developed ;  and  it  is  stated  that  the  olfactory  structures  are 
paired  at  first,  the  azygos  condition  of  the  adult  being  a  secon- 


FIG.  226. 
Egg  of  Bdello- 
stotna;  at  a ,  a 
single  hook 
enlarged. 


FIG.  227.     Palceospondylus  (enlarged),  from  Dean,  after  Traquair. 


dary  feature.  The  young  hatches  from  the  egg  in  a  larval  con- 
dition known  as  the  ammoccetes  stage,  with  rudimentary  eyes 
and  a  large  hood-shaped  upper  lip.  This  is  later  metamor- 
phosed into  the  adult. 

Fossil   marsipobranchs  are  imperfectly  known.     Formerly 


CYCLOSTOMES.  22$ 

peculiar  structures  known  as  conodonts  were  regarded  as  myx- 
inoid  teeth,  but  later  these  have  been  supposed  to  be  annelid 
jaws.  Traquair  has  recently  described,  under  the  name  Palao- 
spondylus  gtmni,  a  fossil  from  the  Devonian  of  Scotland,  which 
may  prove  to  be  a  palaeozoic  marsipobranch,  or  possibly  the. 
larva  of  some  higher  form. 

SUB-CLASS  I.    PETROMYZONTES  (HYPEROARTIA). 

Marsipobranchs  with  well-developed  dorsal  fins  ;  hypophysial 
duct  closed,  its  external  opening  on  the  top  of  the  head ;  seven, 
branchial  openings  on  either  side ;  branchial  basket  well  devel- 
oped ;  pharyngeal  region  divided  by  a  longitudinal  partition  into 
a  dorsal  food  tube  and  a  ventral  respiratory  duct  from  which  the 
gill  slits  arise ;  a  slightly  developed  spiral  valve  in  the  intestine. 

The  lamprey  eels  live  both  in  salt  and  in  fresh  water,  some 
of  the  marine  species  ascending  rivers  in  the  spring  to  lay  their 


FIG.  228.     See  lamprey,  Petromyzon  marinus,  after  Goode. 

eggs.  When  ovipositing  they  attach  themselves  to  stones,  or 
take  up  smaller  pebbles  with  their  suckers  to  make  their  nests, 
a  fact  which  is  reflected  in  the  generic  names  Petromyzon  and 
Lampetra.  The  lampreys  feed  upon  the  mucus  and  blood  which 
they  rasp  from  fishes  to  which  they  attach  themselves.  The 
lampreys  are  included  in  a  single  family,  PETROMYZONID.E,  and 
are  grouped  in  several  genera. 

Petromyzon,  second  dorsal  joined  to  caudal,  supraoral  tooth  with  2-3: 
cusps.  P.  marinus  (sea  lamprey),  Europe  and  North  America  —  Atlantic. 
Lampetra,  smaller  species  (brook  lampreys),  with  broad  supraoral  tooth 
with  median  cusp  small  or  lacking.  L.  planeri,  Europe.  Scarcely  different 
is  L.  wilder i  from  New  York.  Other  genera  are  Mordacia  and  Geotria? 
from  the  southern  hemisphere. 


224  CLASSIFICATION  OF   VERTEBRATES. 

SUB-CLASS    II.     MYXINOIDEI    (HYPEROTRETIA). 

Dorsal  fin  small  or  absent,  hypophysial  duct  opening  at  tip 
of  snout,  posteriorly  communicating  with  mouth  cavity.  Bran- 
chiae 6-14  pairs,  the  pharyngeal  region  not  divided.  Behind 
the  gill  clefts  on  the  left  side  occurs  an  oesophageo-cutaneous 
duct,  connecting  the  pharynx  with  the  exterior.  Branchial  bas- 
ket rudimentary,  snout  bearing  eight  barbels  ;  spiral  valve  absent. 

The  hag-fishes  and  borers  are  the  nearest  approach  to  para- 
sites in  the  vertebrate  phylum.  They  fasten  themselves  to  the 
gill  region  of  fishes,  and  work  their  way  into  the  inside,  where  they 
rapidly  devour  the  flesh,  leaving  merely  a  hulk  of  skin  and  bones. 

Family  MYXINID.E.  Branchiae  6,  the  clefts  uniting  external  to  the 
branchial  pouches  to  open  to  the  exterior  by  a  single  opening.  Myxine, 
the  only  genus.  M.  glutinosa,  in  the  northern  Atlantic  south  to  Cape  Cod. 
Family  BDELLOSTOMID^.  Gills  6-14,  each  with  its  own  duct.  Bdellostoma 
(Heptatreina,  Polistotrema],  the  only  genus,  occurs  in  the  Pacific,  where 
J3.  dombey  ranges  from  California  to  Chile. 

OSTRACODERMI. 

The  ostracoderms  are  a  group  of  fish-like  forms  from  palaeozoic  rocks 
of  exceedingly  doubtful  relations.  They  have  been  regarded  as  ganoids, 
as  cyclostomes,  as  tunicates,  and  even  as  having  relations  to  the  xiphosures 
(Limulus).  This  uncertainty  of  position  is  due  to  the  fact  that  no  traces 
of  jaws  or  of  internal  skeleton  have  yet  been  found.  The  anterior  half  of 
the  body  was  enveloped  in  an  exoskeleton  of  large  bony  plates  which  no 


FIG.  229.     Restoration  of  Pteraspis,  after  Lankester. 

one  has  yet  satisfactorily  homologized  with  the  bones  in  any  modern  form. 
The  hinder  part  of  the  body  was  fish-like,  the  tail  heterocercal.  Frequently 
the  anterior  plates  bear  traces  of  canals  supposed  to  have  contained  lateral 
line  organs,  while  the  head  region  contained  pits  which  may  have  been 
occupied  by  eyes. 

ORDER   I.     HETEROSTRACI. 

Head  region  covered  above  by  a  few  firmly  united  plates,  below  by  a 
single  ventral  plate  ;  tail  sometimes  scaled  ;  orbits  lateral ;  no  paired  appen- 
dages. Pteraspis,  the  best-known  genus,  comes  from  the  Devonian. 


GNA  THOSTOMES. 


225 


ORDER   II.     ASPIDOCEPHALI 
(OSTEOSTRACI). 

Head  covered  by  a  large,  simple,  arcuate 
shield  much  like  the  cephalothorax  of  Limulus 
in  outline,  with  'orbits'  near  its  centre.  Tail 
covered  with  rhomboid  scales  of  varying  size. 
No  paired  appendages  known.  Cephalaspis, 
Devonian.  Auchenaspis,  upper  Silurian. 

ORDER    III.     ANTIARCHA. 

Head  and  trunk  covered  with  large  polygo- 
nal plates  coated  with  enamel ;  tail  with  small 
scales,  '  orbits '  dorsal,  close  together,  a  single 
pair  of  appendages  ('pectoral  fins'),  covered 
with  strong  plates,  and  each  jointed  near  the 
middle.  Pterichthys  (lower  Devonian),  1-4 
inches  long.  Asterolepis,  6  inches  long  (Devo- 
nian). Bothriolepis. 


M 


FlG.  230.    Cephalaspis, 
from  Dean,  after  Agassiz. 


FIG.  231.  Restoration  of  Pterichthys  testudinarius,  after  Traquair.  ADL, 
anterior  dorso-lateral ;  AMD,  anterior  median  dorsal;  AVL,  anterior  ventro- 
lateral ;  EL,  extra  lateral  (opercular);  /.,  labial;  AfO,  median  occipital;  PDL, 
posterior  dorso-lateral;  PM,  premedian ;  PAID,  posterior  median  dorsal;  PVLy 
posterior  ventro-lateral ;  SL,  semilunar.  Lateral  line  system  dotted. 


Series  II.     Gnathostomata. 

Vertebrates  with  jaws;  paired  appendages  normally  present 
olfactory  organs  paired,  and  not  connected  with  hypophysis 
optic  chiasma  present  ;  three  semicircular  canals  in  the  ear 
gills,  when  present,  never  more  than  seven  pairs. 

The  gnathostomes  embrace  the  great  majority  of  verte- 
brates, and  the  account  of  that  group  given  in  the  first  part  of 
this  volume  applies  largely  to  these  forms.  The  mouth  is  trans- 
verse ;  paired  limbs  occur  in  all,  except  where,  as  in  certain 


226  CLASSIFICATION  OF   VERTEBRATES. 

eels,  snakes,  etc.,  they  have  been  lost.  The  skeleton  is  well 
developed,  the  vertebrae  being  cartilaginous  or  osseous,  while  the 
skull  is  more  or  less  completely  roofed  in  with  cartilage  or  bone. 
The  gill  slits  are  narrow  clefts,  and  are  never  more  than  seven 
in  number,  and  the  branchial  arches  are  well  developed.  The 
gnathostomes  are  subdivided  into  two  great  divisions  or  grades, 
—  the  Ichthyopsida  and  the  Amniota. 

GRADE   I.     ICHTHYOPSIDA   (ANAMNIA, 
ANALLANTOIDEA). 

As  the  name  implies,  the  ichthyopsida  include  the  fish-like 
forms  characterized  by  the  presence  of  functional  gills,  either 
in  the  larval  stages  or  throughout  life,  the  absence  of  an 
amrnon  and  true  allantqis  (see  amniota),  and  the  presence  in 
young  or  adult  of  median  fins. 

The  surface  of  the  body  is  rich  in  glands,  and  in  most  forms 
the  skin  contains  scales  largely  of  dermal  origin  (p.  92).  In 

the  young  the  skin  also  bears 
sense  organs  belonging  to  the 
lateral  line  system  (p.  67)  in- 
nervated by  the  seventh  and 
tenth  nerves ;  but  in  those 
forms  in  which  the  adults  as- 
FIG.  232.  Diagram  of  skin  of  a  fish,  sume  a  terrestrial  life  (am- 

c,  cuticle ;  ,/,  dermis;  ,,  epidermis,  con-  phibia)  thig  s  stem  becomes 
taming,  g,  epidermal  glands. 

lost   upon   leaving    the    water. 

In  the  young,  median  fins,  formed  by  a  fold  of  the  integument, 
occur ;  and  in  the  fishes  these  are  supported  by  rays  of  dermal 
origin,  or  by  true  skeletal  rays,  or  by  both.  In  the  amphibia 
the  rays  are  lacking,  and  the  fins  themselves  disappear  on  the 
assumption  of  a  terrestrial  life. 

In  the  skeleton  the  noticeable  features  are  the  small  size,  or 
absence,  of  the  basisphenoid,  and  the  large  size  of  the  parasphen- 
oid  when  present.  Usually  (fishes)  no  sternum  is  present  ;  in 
the  amphibia,  where  it  occurs,  it  is  never  connected  with  the 
ribs.  The  branchial  arches  are  four  or  more  in  number,  and 
these  are  largely  persistent  in  the  adult. 


FfSffES. 


227 


In  the  brain  the  separate  divisions  are  subequal  in  size. 
The  vagus  nerve  innervates  all  but  the  anterior  pair  of  gill  slits, 
and  in  the  aquatic  forms  bears  a  large  ramus  lateralis  distrib- 
uted to  the  sense  organs  of  the  skin.  The  eleventh  nerve  is 
apparently  a  branch  of  the  vagus,  while  the  twelfth  is  repre- 
sented by  the  first,  or  first  and  second,  spinal  nerves. 

The  heart,  usually  far  forward  in  position,  has  the  sinus 
venosus  external  to  the  atrium,  the  atrium  single  or  divided  by  a 
longitudinal  septum  into  right 
and  left  auricles,  the  ventricle 
always  simple  and  undivided. 
In  the  lower  forms  the  conus 
is  large  and  well  developed ;  in 
the  teleosts  it  is  reduced  to  a 
row  of  valves  between  the  ven- 
tricle and  the  bulbus.  At  least 
one  pair  of  aortic  arches  per- 
sists in  a  complete  state  in 
the  adult,  while  some  or  all  are  A 

permanently     or     temporarily          FIG   233-    Relations  of  the  conus, 

J  l  J        A,  in   elasmobranchs   and   ganoids;    />, 

(amphibia)    connected     with       in    teleosts.     a,   auricle;    />,    bulbus    ar- 
the  gills.       A   renal    portal  Sys-       teriosus;    r,  conus,   reduced   in  B  to   a 

tern  (p.  194)  occurs  ;  and  the     circle  of  valves;  *•  ventncle' 
red  blood  corpuscles  are  1-arge,  oval,  and  nucleated.     The  func- 
tional kidney  is  the  mesonephros,  although  rarely  the  proneph- 
ros   persists. 

The  alimentary  canal  is  comparatively  short,  and  either 
terminates  in  a  cloaca  (p.  39),  or  the  vent  is  anterior  to  the 
urogenital  openings.  No  metanephros  is  developed,  and  the 
ova  are  frequently  large. 

The  Ichthyopsida  are  divisible  into  Pisces  and  Amphibia. 

CLASS  I.     PISCES. 

Ichthyopsida  with  persistent  gills  ;  paired  appendages  al- 
most always  present  in  the  shape  of  fins  ;  median  fins  sup- 
ported by  dermal  rays ;  body  usually  covered  with  dermal 
scales  ;  postcava,<Eustachian  tube,  and  stapes  lacking.  Nos- 
trils (except  in  dipnoi)  never  opening  into  the  mouth. 


228  CLASSIFICATION  OF   VERTEBRATES. 

Under  the  heading  Pisces  are  included  all  those  forms, 
except  the  lampreys  and  hags,  commonly  known  as  fishes. 
The  group  is  well  marked  off  from  all  other  groups  of  verte- 
brates to-day,  and  the  only  questions  of  classification  are  those 
pertaining  to  the  relationships  and  arrangement  of  the  various 
groups  composing  the  class. 

The  skin  usually  contains  abundant  gland  cells  secreting 
mucus,  but  multicellular  glands  are  rare.  The  body  is  usually 
covered  with  scales  of  dermal  origin  ;  but  these  are  occasionally 
absent,  as,  for  instance,  in  most  electrical  fishes.  Some  years 
ago  the  scales  were  of  importance  in  classification,  and  four 
types  were  recognized  :  (i),  placoid  scales,  occurring  in  the 
elasmobranchs,  in  which  there  is  a  basal  plate. of  dentine  bearing 
a  central  spine  (p.  92)  of  dermal  origin,  the  spine  being  tipped 
with  ectodermal  enamel.  This  type  of  scale  is  regarded  as  the 
most  primitive,  and  as  having  given  rise  to  teeth  and  dermal 
bones  (p.  164).  (2),  Ganoid  scales  ;  large,  bony  (dermal)  rhom- 
boidal  plates  embedded  in  the  skin,  and  frequently  bearing  on 
their  outer  surface  a  layer  of  enamel  (ganoin),  likewise  of  dermal 
origin.  This  type  of  scale  is  found  in  most  ganoids,  and  was 
especially  characteristic  of  the  early  members  of  this  group. 
(3),  Cycloid  scales,  more  horny  in  character,  lacking  in  enamel, 
and  embedded  in  dermal  pockets ;  these  have  the  outline 
approximately  circular.  (4),  Ctenoid  scales  differ  from  these 
last  in  having  small  spines  upon  the  posterior  or  free  margin. 
Occasionally  scales  may  fuse  to  form  large  bony  plates  or 
scutes. 

Median  fins  are  always  present,  and,  except  in  degenerate 
forms,  two  pairs  of  paired  fins  as  well.  These  are  supported  by 
rays  of  dermal  origin,  and  the  paired  fins  also  have  cartilagi- 
nous or  bony  basal  supports  (p.  177).  In  the  young  the  median 
fins  are  continuous  around  the  tail,  and  this  condition  persists 
in  the  adult  of  a  few  forms  (e.g.,  eels)  ;  but  usually  it  is  inter- 
rupted so  that  we  may  recognize  fins  on  the  back  (dorsals),  on 
the  tail  (caudal),  and  on  the  ventral  surface  behind  the  vent 
(anal).  The  shapes  of  these  and  the  number  of  supporting 
rays  are  of  importance  in  the  discrimination  of  species,  etc.,  but 
more  important  are  some  of  the  peculiarities  of  the  caudal  fin. 


FSSHES. 


229 


In  the  primitive  condition  the  vertebral  axis  continues 
straight  to  the  tip  of  the  tail,  the  caudal  fin  being  developed 
symmetrically  on  either  side  (diphycercal  fin,  Fig.  234,  A). 
This  is  found  in  but  few  forms.  In  elasmobranchs  and  many 
ganoids  the  heterocercal  type  occurs  (Fig.  234,  B).  Here  the 


FIG.  234.  Different  forms  of  tails  of  fishes,  from  Hertwig  after  Zittel.  A, 
diphycercal  {Pofypterus)\  B,  heterocercal  {Acipenser}\  C,  homocercal  (Atnia}; 
D,  homocercal  (Sal/no}. 

axis  of  the  body  is  bent  abruptly  upwards  ;  and  besides  the 
caudal  of  the  diphycercal  condition  a  secondary  lobe  develops 
on  the  lower  side,  giving  the  tail  an  unsymmetrical  appearance. 
The  homocercal  condition  (Fig.  234,  C  and  D)  is  derived  from 
this  last  by  the  greater  development  of  the  ventral  lobe,  so  that 
the  fin  appears  symmetrical  externally,  although,  as  will  be  seen 
from  the  figures,  the  skeletal  parts  are  of  the  heterocercal  type. 


230  CLASSIFICATION-  OF   VERTEBRATES. 

These  same  figures  also  illustrate  another  point.  The  rays  of 
the  ventral  side  of  the  fin  are  supported  upon  the  haemal  arches 
(Fig.  234,  D)  or  upon  elements  (interspinalia)  alternating  with 
these  (Fig.  234,  A),  while  those  of  the  dorsal  side  (and  this 
applies  also  to  the  dorsal  and  anal  fins)  are  borne  on  similar 
interspinalia.1 

The  alimentary  canal,  though  rarely  straight  (holocephali, 
dipnoi,  and  some  teleosts),  does  not  present  such  convolutions  as 
are  common  in  the  higher  groups.  Teeth  are  almost  univer- 
sally present  (except  in  sturgeon2  and  some  lophobranchs),  but 
salivary  glands  never  occur.  The  stomach  is  usually  not 
sharply  marked  off  from  the  oesophagus,  and  it  frequently  has 
a  distinct  siphon  or  U-shape. 

Gills  are  always  present,  the  clefts,  usually  five  in  number 
(six  or  seven  in  diplospondyli  and  some  fossil  forms;  four  in 
holocephali).  Besides  these  the  hyomandibular  cleft  persists 
in  most  elasmobranchs  and  ganoids  as  a  much  smaller  opening, 
the  spiracle,  usually  placed  on  the  top  of  the  head.  In  many 
elasmobranchs,  teleosts,  and  dipnoi  the  gill  arches  bear  gill 
rakers,  —  small  conical  cartilages  which  extend  into  the  clefts. 
In  the  young  elasmobranchs,  sturgeon,  and  some  teleosts  the 
gills  extend  outside  the  clefts  as  long  fringes  in  the  young. 
Persistent  external  gills  occur  in  Polypterus,  and  occasionally  in 
Protopterns,  while  they  are  retained  for  some  time  in  other 
forms. 

In  the  adult  fish  the  brain  occupies  but  a  part  of  the  cra- 
nial cavity.  It  is  characterized  by  the  nearly  complete  disap- 
pearance of  flexures  in  the  adult  as  viewed  from  the  outside, 
and  by  the  slight  development  of  the  cerebral  cortex,  which 
in  the  teleosts  is  lacking,  being  represented  by  a  thin  epi- 
thelial layer,  the  cerebrum  consisting  in  this  group  of  only 
the  corpora  striata  of  higher  forms.  The  olfactory  lobes  are 
distinct,  and  either  lie  close  to  the  cerebrum  (most  teleosts) 
or  are  removed  some  distance,  in  which  case  they  are  con- 

1  If  the  median  or  azygos  fins  have  arisen  by  the  coalescence  of  pairs  of  lateral  folds 
(p.  1 73),  these  interspinalia  would  then  correspond  to  the  basalia  of  the  paired  fins,  as  main- 
tained by  Dohrn  and  Mayer. 

2  The  embryonic  sturgeon  has  teeth. 


FISHES.  231 

nected  with  the  brain  by  an  elongate  tractus  olfactonus.  The 
optic  thalamic  are  small,  but  the  lobi  inferiores  on  the  ventral 
surface  of  the  twixt  brain  are  well  marked.  The  optic  lobes 
are  strongly  developed  (only  a  single  lobe  occurs  in  Proto- 
pterns),  and  contain  a  large  ventricle  (epicoele).  The  optic 
nerves  cross  in  the  teleosts  ;  a  true  chiasma  is  developed  in 
other  groups  (p.  61).  The  eleventh  and  twelfth  nerves  are 
lacking. 

The  nasal  organs  only  exceptionally  have  connection  with  the 
cavity  of  the  mouth.  The  eyes  are  without  accessory  glands  ; 
the  lens  is  strongly  convex,  and  is  connected  with  the  wall  of  the 
posterior  chamber  by  a  structure,  the  falciform  process,  the  distal 
end  of  which  is  swollen  into  a  bulb,  the  campanula  Halleri,  in 
the  walls  of  which  muscles  are  developed  so  that  this  structure 
plays  a  part  in  accommodation.  There  is  no  true  Eustachian 
tube  in  connection  with  the  ear,  and  a  stapes  never  occurs. 

The  muscular  system  shows  plainly  its  division  into  myo- 
tomes,  and  there  is  no  differentiation  of  layers  of  oblique  muscles 
in  the  abdominal  walls.  Epiaxial  and  hypaxial  muscles  (p.  109) 
are  well  differentiated. 

The  vertebral  centres  are  either  incompletely  developed  or 
they  are  of  the  amphicoelous  type  ;  the  only  exception  being 
found  in  Lepidosteus,  where  they  are  opisthoccelous,  this  con- 
dition being  brought  about  in  the  same  way  as  in  amphibia 
(p.  139).  Only  trunk  and  caudal  regions  are  differentiated. 
Ribs,  when  present,  are,  except  in  elasmobranchs,  modified 
haemal  arches.  In  Polypterus  both  kinds  of  ribs  occur  (p.  144). 
The  skull  is  noticeable  for  the  great  development  of  the  visceral 
arches  and  their  independence  from  the  other  parts.  Almost 
always  the  hyomandibular  acts  as  a  suspensor  of  the  lower  jaw. 
The  paired  appendages  are  always  fins,  pectoral  and  ventral,  but 
occasionally  one  or  the  other  of  these  may  be  degenerate.  As 
a  rule  these  occupy  positions,  the  pectoral  at  what  would  compare 
to  the  shoulder,  the  ventral  just  in  front  of  the  anus  ;  but  occa- 
sionally the  ventral  fins  may  move  forward  to  a  position  just 
behind  the  pectorals  (thoracic),  or  even  in  a  line  with  these 
(jugular).  The  pelvic  girdle  never  enters  into  connection  with 
the  vertebral  column  ;  the  shoulder  girdle  rarely.  The  latter, 


232  CLASSIFICATION  OF   VERTEBRATES. 

however,  may  become  connected  (many  teleosts)  with  the  otic 
region  by  a  chain  of  bones.  The  skeleton  of  the  fins  themselves 
varies  greatly,  but  there  is  never  anything  that  approaches  the 
pentadactyle  condition  found  in  the  higher  vertebrates. 

The  heart  always  consists  of  a  single  atrium  and  a  single 
ventricle,  and  a  sinus  which  is  undivided  except  in  the  dipnoi. 
Conus  and  bulbus  vary  in  their  development.  The  aortic  arches 
are  always  symmetrically  developed.  No  postcava  occurs  ;  the 
blood  from  the  posterior  portion  of  the  body  being  returned  by 
the  postcardinals,  the  portal  system,  and  the  hypogastric  veins. 
A  spleen  is  always  present. 

The  functional  excretory  organ  is  the  mesonephros,  the  pro- 
nephros  but  rarely  persisting.  Urinary  bladders  are  frequently 
present ;  but  these  are  simply  expansions  of  the  urinary  duct, 
and  are  not  comparable  to  the  allantoic  bladder  of  the  higher 
vertebrates.  The  urinary  ducts  either  empty  into  the  hinder 
end  of  the  intestine  or  by  separate  openings  to  the  exterior. 
Pori  abdominales  are  almost  always  present,  and  these  in  some 
cases  serve  for  the  extrusion  of  the  reproductive  products.  Only 
in  elasmobranchs  and  ganoids  do  the  urogenital  ducts  serve  to 
carry  away  the  eggs  and  milt. 

SUB-CLASS  I.     ELASMOBRANCHII   (PLAGIOSTOMI, 
CHONDROPTERYGII).     . 

Cartilaginous  fishes  without  true  bones  ;  tail  usually  hetero- 
cercal  ;  gill  slits  5-7,  no  operculum  present ;  skin,  with  rare  ex- 
ceptions, bearing  placoid  scales  ;  pelvic  fin  in  males  of  recent 
forms  bearing  a  complicated  copulatory  apparatus  (  '  clasper '  )  ; 
skull,  amphi-  or  hyostylic,  never  autostylic ;  upper  jaw  formed 
by  pterygoquadrate  ;  a  true  optic  chiasma ;  intestine  with  a  spiral 
valve ;  no  air-bladder. 

The  sub-class  elasmobranchii  includes  those  fishes  popularly 
known  as  sharks  and  skates,  all  of  which, -with  two  or  three 
exceptions,  are  marine.  The  body  is  usually  fish-like  in  shape, 
but  is  greatly  flattened  horizontally  in  the  skates.  The  caudal 
fin,  when  present,  is  always  heterocercal ;  but  in  many  skates  the 
fin  is  absent,  the  tail  tapering  to  a  point.  In  most  forms  the 
mouth  and  nostrils  are  ventral  in  position,  and  are  placed  some 


ELASMOBRANCHS. 


233 


distance  behind  the  tip  of  the  snout.  The  body  is  covered  with 
placoid  scales,  which  are  usually  small,  and  form  shagreen,  for- 
merly much  used  by  cabinet-makers  in  place  of  sandpaper.  The 
pelvic  fins  are  always  abdominal  in  position. 

The  jaws  are  armed  with  acutely  pointed  teeth  or  with  flat- 
tened crushing-plates.  The  oesophagus  is  ciliated  ;  the  stomach 
shaped  like  the  letter 
J,  and  no  pyloric  caeca 
occur.  The  intestine 
is  provided  with  a  well- 
developed  spiral  valve, 
and  the  rectum  bears 
a  finger-shaped  rectal 
gland  on  its  dorsal  sur- 
face. A  cloaca  is  pres- 
ent. 

The  gill  clefts  are 
usually  five  in  number, 
six  or  seven  in  some 
lower  forms.  They 
open  freely  to  the  out- 
side, no  operculum  be- 
ing developed.  The 
gills  are  attached  their 
whole  length  to  the 
interbranchial  septum. 
Usually  a  spiracle  oc-  FlG-  235-  Relations  of  gill  clefts  in  the  elas- 

,    .  .  ,  mobranchs.      //,    hyoid    arch;    m,    mandible;    s9 

curs,  and  this  may  bear      spirade> 
a  pseudobranch  (p.  23). 

The  hemispheres  of  the  brain  are  united,  and  the  olfactory 
lobes  are  separated  from  the  cerebrum  by  an  elongate  olfactory 
tract.  The  twixt  brain  is  short,  and  an  optic  chiasma  occurs. 
The  lateral  line  system  is  well  developed,  and  in  the  skates  be- 
comes greatly  branched.  On  the  head  are  numerous  sensory 
ampullae  filled  with  jelly. 

The  skeleton  is  cartilaginous  ;  but  in  many  ci|ses  it  is  ren- 
dered more  dense  by  the  deposition  of  lime,  which,  however, 
never  takes  the  shape  of  bone  corpuscles,  there  being  a  sharp 


234 


CLASSIFICATION  OF   VERTEBRATES. 


FIG.  236.  Diagrams  of  cyclospondylous  {A} 
and  asterospondylous  (Z?)  vertebrae.  Calcifications 
of  cartilage  black. 


line  between  calcified  cartilage  and  bone.     Membrane  bones  are 
absent  in  all  recent  forms,  but  in  some  fossils  the  dermal  scales 

united  to  form  an  ex- 
tensive armor.  In 
some  the  vertebral 
centra  are  entirely  of 
cartilage.  When  lime 
is  deposited  in  them 
it  may  take  two 
shapes,  either  laid 
down  in  concentric 
areas  (cyclospondy- 
lous type),  or  in  a  ra- 
dial manner  (astero- 
spondylous). In  the  diplospondyli  and  in  the  tail  of  some  skates 
an  embolomerous  condition  occurs.  In  all  recent  forms  the  neu- 
ral arch  is  converted  into  a  closed  canal  by  the  insertion  of 
intercalary  pieces  betweeen  the  neural  processes  and  spine. 

The  cranium  is  a  solid  box  without  sutures.  In  its  roof 
there  may  be  one  or  two  gaps  (f ontanelles)  closed  by  membrane. 
The  pterygoquadrate  is  never  firmly  united  to  the  cranium,  but 
•either  articulates  directly  with  it  (amphistylic,  Fig.  237),  or  is 
supported  by  ligaments 
and  by  the  interven- 
tion of  the  hyomandib- 
ular  between  the  hinder 
end  of  the  pterygoquad- 
rate and  the  otic  region 
of  the  cranium  (hyo- 
stylic,  Fig.  162),  thus 
forming  a  suspensor 
for  the  jaws.  The 
pterygoquadrate  forms 
the  upper  jaw,  but  is  re- 
enforced  in  many  spe- 
cies by  labial  cartilages.  In  some  extinct  elasmobranchs  girdles 
are  apparently  absent,  but  in  all  recent  forms  they  are  well 
developed.  The  pectoral  girdle  consists  of  a  simple  U-shaped 


FIG.  237. 


Skull  of  Heptanchus,  after  Giinther ; 
amphistylic. 


ELASMOBRA  NCHS.  235 

arch,  usually  free,  but  occasionally  (raiae)  connected  with  the 
vertebral  column,  or  rarely  with  the  skull.  The  pelvic  girdle 
consists  of  a  transverse  ventral  bar  without  dorsal  or  iliac 
processes. 

A  con  us  with  two  or  more  rows  of  valves  occurs  in  connec- 
tion with  the  heart  ;  the  aortic  arches  and  the  chief  arteries  and 
veins  are  of  the  primitive  type,  and  a  cardinal  sinus  (p.  195) 
usually  occurs. 

The  eggs  are  few  and  large,  and  in  the  recent  forms  undergo 
internal  inpregnation,  more  or  less  complicated  structures  (the 
'  claspers  '),  which  serve  as  intromittent  organs,  being  developed 
in  the  pectoral  fins  of  the  male.  The  spermatozoa  is  carried  to 
the  exterior  by  means  of  the  Wolffian  duct,  while  the  Miillerian 
duct  serves  as  oviduct,  the  fused  nephrostomes  of  the  rudimen- 
tary pronephros  serving  as  the  ostium  tubae.  In  many  sharks 
and  in  some  of  the  rays  a' portion  of  the  oviduct  becomes  enlarged 
into  a  uterus,  and  in  some  species  of  CarcJiarias  and  Mustelns,  a 
placenta  is  formed,  at  first  sight  strikingly  similar  to  that  of  the 
mammals,  but  developed  from  the  yolk  sac  rather  than  from  an 
allantois  (see  mammals).  The  outer  surface  of  the  yolk  sac 
in  these  forms  becomes  richly  vascular  ;  and  this  becomes  con- 
nected with  the  uterine  walls,  so  that  the  growing  embryo 
receives  nourishment  from  the  blood  of  the  mother. 

The  segmentation  is  restricted  to  a  portion  of  the  upper 
surface,  i.e.,  is  meroblastic.  The  result  of  this  is  the  formation 
of  a  circular  layer  of  cells,  the  blastoderm,  resting  upon  the  yolk. 
Inside  of  these  cells  is  a  space  corresponding  to  the  segmen- 
tation cavity  of  the  typical  egg  (p.  211).  Owing  to  the  great 
amount  of  yolk,  the  process  of  gastrulation'  becomes  greatly 
modified.  At  one  end  of  the  blastoderm  the  cells  turn  in  be- 
tween the  blastoderm  and  the  yolk,  and  these  ingrowing  cells 
become  the  entoderm.  At  the  place  where  this  ingrowth  occurs 
an  arcuate  elevation  appears,  terminating  in  a  pair  of  swell- 
ings, the  tail  swellings,  on  the  margin  of  the  blastoderm.  With 
growth  the  distance  between  the  arched  elevation  (which  marks 
the  tip  of  the  head)  and  the  tail  swellings  increases,  forming 
the  rest  of  the  head  and  the  trunk  region  of  the  body.  At  first 
this  embryonic  area  forms  a  broad,  shallow  medullary  plate  ;  but 


236 


CLASSIFICATION  OF   VERTEBRATES. 


FIG.  238.  Section  through  the 
broadly  expanded  medullary  plate 
of  a  shark  {Acantkias}.  a, 
archenteron  ;  c,  ccelom  ;  m,  meso- 
thelium  ;  w/,  medullary  plate  ;  n, 
notochord. 


the  edges  of  this  rise  up  and  gradually  unite,  so  that  the  plate 
becomes  converted  into  the  medullary  tube. 

Gradually  the  embryo  thus  out- 
lined is  raised  above  the  yolk,  and 
soon  becomes  so  separated  from  it 
that  only  a  slender  yolk  stalk  re- 
mains connecting  the  two.  This 
stalk  carries  blood-vessels  (ompha- 
lomesaraics),  while  the  yolk  itself 
is  connected  with  the  alimentary 
canal. 

The  mesothelium  arises  as  in- 
growths on  either  side  at  the  point 
of  differentiation  of  ectoderm  and 
entoderm,  these  growing  in  be- 
tween the  two  layers.  The  gill  slits  break  through  the  sides 
of  the  neck  in  regular  succession  from  in  front  backwards,  the 
mouth  breaking  through  after  all  five  gill  slits  are  open.  For 
a  time  the  gill  filaments  protrude  from  the  gill  slits.  The  spi- 
racle is  at  first  the  largest  of  the  clefts,  but  it  soon  begins  to 
close  at  the  ventral  end  so  that  only  the  dorsal  portion  persists. 
The  paired  fins  start  as  lateral  folds  (in 
some  cases  continuous,  e.g.,  Acanthias), 
into  which  grow  cells  from  the  myotomes 
(p.  no). 

Elasmobranchs  are,  on  the  whole,  the 
most  primitive  of  the  jawed  vertebrates, 
although  in  some  respects  they  seem  to 
stand  above  the  other  fish-like  forms. 
The  sharks  are  free-swimming  forms,  seek- 
ing their  prey  in  all  parts  of  the  sea,  a 
few  species  ascending  rivers,  and  one  / 
being  found  in  Lake  Nicaragua.  The 
skates,  on  the  other  hand,  are  bottom 
feeders,  living  on  molluscs,  crabs,  etc., 
and  their  teeth  are  modified  into  crush- 
ing-plates adapted  to  such  food.  None  of  the  species  are  very 
small ;  but  some  of  them  are  of  enormous  size,  among  the  largest 


FIG.  239.  Head  of 
embryo  Acanthias,  show- 
ing the  gill  filaments  pro- 
truding from  the  gill  slits. 


ELASMOBRANCHS. 


237 


of  living  fishes ;  sharks  reaching,  in  the  basking  shark  and  blue 
shark,  a  length  of  thirty-five  or  forty  feet ;  the  rays,  in  Mania, 
a  breadth  of  fifteen  feet. 

ORDER    I.     CLADOSELACHII    (PLEUROPTERYGII). 

Notochord  persistent ;  neural  and  haemal  arches  slender. 
Paired  fins  with  basalia  and  radialia  arranged  much  as  in  the 
median  fins  of  recent  fish  (Fig.  188).  No  claspers  yet  found. 


FIG.  240.     Lateral  and  ventral  veins  of  Cladoselache,  restored  by  Dr.  Dean. 

Apparently  a  flap  of  skin  much  like  an  operculum  covered  the 
first  of  the  gill  slits  which  were  seven  (possibly  nine)  in  num- 
ber. Jaws  apparently  hyostylic.  Lateral  line  an  open  groove. 
The  only  known  member  of  the  group  is  CladoselacJie,  from 
the  Waverley  group  (lower  carboniferous)  of  Ohio.  It  is  prob- 
able that  some  of  the  fossils  with  similar  teeth  (Cladodus) 
belong  here. 

ORDER    II.     ICHTHYOTOMI. 

Notochord  persistent  ;  neural  and  haemal  arches  and  inter- 
calary cartilages  present.  Pectoral  fin  archipterygial  (p.  172). 
Pelvic  fins  with  claspers,  caudal  fin  diphycercal.  No  placoid 
scales,  but  the  head  was  covered  with  dermal  bones.  The  best- 
known  genus  is  Plenracanthus  (=Xenacanthiis,  Didymodns) 
from  the  carboniferous  and  Permian  of  Europe  and  America. 


238  CLASSIFICATION  OF   VERTEBRATES. 

ORDER    III.     SELACHII. 

Elasmobranchs  with  the  notochord  more  or  less  completely 
replaced  by  vertebral  centra.  The  neural  canal  completely 
arched  in  by  neurapophyses  and  intercalaria.  No  dermal 
bones.  Paired  fins  never  archipterygial ;  claspers  always  de- 
veloped in  the  male.  To  this  order  belong  all  recent  as  well 
as  many  fossil  elasmobranchs. 

SUB-ORDER  i.     DIPLOSPONDYLI. 

Selachians  with  embolomerous  vertebrae  with  two  neural  arches  to  each 
myotomic  centre  ;  a  single  dorsal  fin  ;  anal  fin  present ;  amphistylic  skull,  and 
gill  slits  6  or  7.  Two  families  are  recognized.  The  CHLAMYDOSELACHID^E 


FlG.  241.      Chlamydoselachus  anguineus,  after  Garman. 

has  an  eel-like  body,  terminal  mouth,  nostrils  on  the  dorsal  surface,  Cla- 
dodus-\\\Lt  teeth,  and  6  gill  slits.  To  it  belongs  the  single  genus  Chlamydo- 
selachus of  the  deeper  portions  of  the  northern  parts  of  the  Atlantic  and 
Pacific.  In  the  NOTIDANID^:  the  body  is  shark-like,  the  spiracle  is  lateral, 
the  teeth  differ  in  the  two  jaws.  The  species  are  viviparous.  Hexanchus 
has  six  gills,  Heptanchus  (Fig.  237)  seven. 

SUB-ORDER  2.     EUSELACHII. 

Vertebrae  normal,  branchial  slits  five  in  number,  skull  hyostylic. 

SECTION  a  —  ASTEROSPONDYLI.  The  vertebrae  in  this  group, 
which  include  the  typical  sharks,  are  asterospondylous ;  two  dorsals  and  a 
single  anal  fin  present.  Here  belong  a  large  series  of  forms.  In  the  CESTRA- 
CIONTID,E,  which  is  represented  by  a  few  Pacific  species  to-day,  but  which 
was  abundant  in  past  times,  the  dorsal  fins  bear  spines,  while  the  jaws  be- 
hind bear  transverse  rows  of  pavement-like  teeth.  In  the  GALEID^:  the 
head  is  normal,  the  spiracles  are  small  or  lacking,  the  last  gill  cleft  is  above 


I  tf--     •    :' 
V 

EL'ASMOBRANCHS.  239 

the  pectoral,  and  a  nictitating  membrane  is  present.  Here  belong  the  dog- 
sharks,  grouped  under  Galeus  and  Mustelus ;  the  largest  of  all  sharks, 
commonly  called  Carcharias  (Charcharinus},  some  of  which  have  man-eat- 
ing reputations;  and  the  tiger  sharks,  Galeocerdo.  In  the  hammer-head 
sharks  (ZYG^:NID/E  or  SPHRYNID.<E)  the  structure  is  much  as  in  the  Galei- 
dse,  except  that  the  sides  of  the  head  bearing  the  eyes  are  produced  into- 
lobes,  giving  the  whole  a  mallet-like  appearance,  In  the  thresher-sharks 
(ALOPIID^E)  the  spiracle  is  lacking,  the  last  gill  cleft  above  the  pectoral,  the 
nictitating  membrane  absent,  and  the  tail  about x  as  long  as  the  rest  of 
the  body.  In  the  LAMNID^E,  including  the  mackerel-sharks  (Lamna]  and 
the  great  white  '  man-eater  '  shark  (Carckarodon\  the  teeth  are  sharp,  the 
spiracles  small  or  absent,  and  the  gill  slits  all  in  front  of  the  pectoral. 

SECTION  /*  —  CYCLOSPONDYLI.  Calcareous  deposits  of  the  verte- 
bral centra  arranged  in  one  or  more  concentric  rings  about  the  axis.  In  the 
SQUALID.E,  including  the  common  dog-fishes  {Acanthias  or  Squalus},  the 
fins  are  normal,  the  spiracles  present,  the  gill  openings  in  front  of  pectoral, 
and  the  dorsal  fins  each  with  a  spine  in  front.  In  the  SQUATINID^E  the 
pectoral  fins  are  very  large,  so  that  the  body  has  more  the  shape  of  a  large 
flattened  disk,  presenting  a  close  appearance  to  the  skates,  except  that  the 
pectoral  has  not  grown  to  the  head.  Here,  too,  belongs  the  family  PRISTIO- 
PHORID^E,  in  which  the  snout  is  prolonged  into  a  long  beak,  armed  with 
teeth  on  either  side.  These  saw-fishes  are  confined  to  the  southern  hemi- 
sphere, but  may  readily  be  distinguished  from  the  common  forms  (which 
are  rays)  by  the  position  of  the  gill  slits. 


SUB-ORDER  3.     RALE    (BATOIDEA). 

Vertebrae  normal,  cyclospondylous ;  gill  slits  ventral  in  position ;  spiracles- 
present;  body  typically  flattened  and  rendered  disk-like  by  the  great  de- 
velopment of  the  pectorals.  This  group  is  greatly  specialized,  and  is 
apparently  derived  from  the  cyclospondylous  EuselachSi. 

In  the  PRISTID^E  there  is  no  sharp  distinction  between  disk  and  tail; 
and  the  rostrum  is  prolonged  into  a  saw,  like  that  of  the  Pristiophorida% 
from  which,  however,  these  saw-fish,  which  belong  to  the  northern  hemi- 
sphere, may  be  distinguished  by  the  position  of  the  gills.  Pristis pectinatus 
occasionally  occurs  on  our  southern  coasts.  The  TORPEDINID^E,  which 
includes  Torpedo,  the  electric  skate,  have  the  body  without  scales,  the  disk 
rounded,  and  the  tail  thick  and  fleshy.  A  single  species,  known  as  the 
'  cramp-fish,'  occurs  occasionally  on  our  shores  south  of  Cape  Cod.  The 
electrical  organ  has  been  described  (p.  115).  In  the  RAJID^E,  which  in- 
cludes our  common  skates,  the  disk  is  more  or  less  rhombic  in  outline,  rough- 
ened by  large  placoid  scales,  two  dorsals  without  serrated  spihes.  Several 
species  of  the  genus  Raja  occur  on  the  coasts.  In  the  TRYGONID^E  belong 
the  sting-rays,  which  have  the  tail  usually  whip-like,  never  two  dorsals,  and 
near  the  base  of  the  tail  one  or  two  serrated  spines,  the  'sting,'  which  can. 


240 


CLASSIFICATION  OF   VERTEBRATES. 


inflict  a  severe  wound.  Our  sting-rays  belong  to  the  genus  Dasyatis.  In 
the  MYLIOBATID^E  the  anterior  ends  of  the  pectorals  are  free  from  the  head, 
forming  ear-like  '  cephalic  fins.'  On  our  southern  coast  and  extending  into 
the  tropics  is  the  genus  Manta,  the  species  of  which  are  known  as  devil- 
fish. These  and  allied  forms  are  among  the  largest  of  fishes. 


m 


FiG.  242.  Saw-fish,  Pris- 
tis  pectinatus,  ventral  view, 
after  Goode. 


FiG.  243.     Common  skate,  Raia  erinaceat 
from  Agassiz  and  Gould. 


ORDER   IV.     HOLOCEPHALI. 

Cartilaginous  fishes  in  which  no  true  bone  is  developed  ;  tail 
heterocercal ;  gill  slits  4,  externally  covered  by  a  membranous 
fold,  so  that  but  one  opening  appears  on  the  outside  ;  skin  naked  ; 
pelvic  fin  of  male  with  'clasper';  skull  autostylic  ;  upper  jaw 
formed  by  pterygoquadrate  ;  optic  chiasma  present  ;  a  spiral  valve 
in  the  intestine  ;  no  air-bladder. 

Three  genera  containing  about  half  a  dozen  species  repre- 
sent this  group  to-day.  The  body  is  much  like  that  of  the 
sharks  ;  but  the  four  gill  slits  are  not  visible  from  the  exterior, 


HOLOCEPHA  LL  24 1 

since  a  membranous  fold  of  the  skin,  the  operculum,  grows 
backwards  over  them,  so  that  but  a  single  aperture  is  visible 
exteriorly.  The  spiracle  is  closed.  The  pelvic  fins  are  abdomi- 
nal in  position  and  bear  claspers  in  the  males,  while  in  front  of 
these  fins  there  is  a  pit  containing  an  anterior  clasper  armed 
with  hook-like  spines,  the  function  of  which  is  unknown.  In 
addition  there  is  borne  on  the  head  in  Chimczra  and  Callor- 
hynchus  a  peculiar  frontal  clasper  differing  in  the  sexes.  The 
skin  in  recent  species  is  naked,  and  the  lateral  line  on  the  trunk 
is  an  open  groove.  The  tail  is  heterocercal. 

The  mouth  is  terminal,  the  nostrils  dorsal,  the  latter  not 
communicating  with  the  mouth.  The  teeth  are  in  the  form 
of  strong  plates,  two  pairs  in  the  upper  jaw,  a  single  pair  in 
the  lower.  These  have  areas  (tritors)  specially  hardened  by 
deposits  of  lime.  The  alimentary  canal  is  almost  straight ;  the 
intestine  possesses  a  spiral  valve.  No  cloaca  is  present,  the 
urogenital  ducts  opening  behind  the  vent.  The  gills  are  re- 
lated to  the  septa  as  in  the  selachians.  No  air-bladder  is 
developed. 

The  brain  has  the  hemispheres  distinct  from  each  other,  the 
olfactory  lobes  being  separated  from  the  cerebrum  by  long 
olfactory  tracts.  The  twixt  brain  is  extremely  long ;  an  optic 
chiasma  occurs.  The  heart  is  like  that  of  selachians,  three 
rows  of  valves  being  present  in  the  conus. 

The  skeleton  is  cartilaginous.  The  notochordal  sheath  con- 
tains cartilage  rings  more  numerous  than  the  segments  ;  and 
the  neural  tube,  composed  of  neural  arches  and  intercalaria,  is 
very  high.  The  cranium  is  high  and  narrow,  and  the  pterygo- 
quadrate  is  indistinguishably  fused  with  the  cranium  (autosty- 
lic).  The  skull  is  movably  articulated  to  the  vertebral  column 
by  a  (basi)  occipital  condyle.  The  shoulder  girdle  is  like  that 
of  elasmobranchs  ;  but  the  pelvic  girdle  consists  of  right  and  left 
halves,  connected  by  ligament.  The  excretory  organs  and  fe- 
.male  reproductive  organs  are  much  as  in  other  elasmobranchs  ; 
the  male  organs  are  noticeable  for  the  large  size  of  the  epi- 
didymes  and  the  seminal  vesicles  developed  from  the  posterior 
portions  of  the  vasa  deferentia.  The  Mullerian  ducts  also  re- 
tain their  lumen  in  the  male,  and  connect  with  the  ccelom  by 


242  CLASSIFICATION  OF   VERTEBRATES. 

a  common  ostium  tubae.  Of  the  development  almost  nothing- 
is  known.  The  eggs  are  very  large,  six  and  one-half  inches 
long  in  Chim&ra,  ten  inches  long  in  Callorhynchus.  Accord- 
ing to  the  unpublished  studies  of  Dr.  Dean,  in  the  early  stages 
these  forms  are  decidedly  shark-like,  and  the  elevation  of  the 
division  to  the  rank  of  a  sub-class  is  not  warranted.  The 
pterygoquadrate  is  free  as  in  other  elasmobranchs  ;  the  gills  are 
not  covered,  and  external  gill  filaments  are  present. 

Only  three  genera  —  Chimcera,  Callorhynchus,  and  Harriotta  —  are 
known  from  existing  seas.  The  first  is  represented  on  our  east  coast  by 
Chimcera  rnonstrosa  and  C.  affinis,  and  on  the  Pacific  by  C.  collei.  Harri- 
otta has  been  found  in  the  North  Atlantic  and  in  Japan.  The  third  genus 


FIG.  244.      Chimcera  monstrosa,  '  king  of  the  herrings.' 

is  from  the  southern  seas,  and  also  occurs  fossil  in  New  Zealand.  Several 
fossil  genera  —  Ischyodus,  Eumylodus,  Rhynchodes,  Edaphodon,  etc.  — 
range  from  the  Devonian  to  the  cretaceous.  The  fossil  Squaloraia  from 
the  lias  of  England  also  appears  to  belong  here.  Less  certain  is  the  group 
PTYCTODOXTID/E  {Ptyctodus,  Rhynchodus}  from  the  Devonian,  known  only 
from  the  dental  plates,  a  pair  in  each  jaw. 

SUB-CLASS   II.     TELEOSTOMI. 

Fishes  in  which  bones  are  developed  ;  gill  slits  5,  exter- 
nally covered  with  a  bony  operculum  ;  scales,  when  present, 
ganoid,  ctenoid,  or  cycloid  ;  no  claspers  ;  skull  hyostylic  ;  upper 
jaw  formed  by  membrane  bones  ;  skull  with  sutures  ;  air-bladder 
frequently  present. 

Here  belong  all  the  common  fishes,  —  trout,  cod,  herring, 
shad,  eels,  etc.,  —  as  well  as  a  series  of  forms  not  so  familiar, 
which  are  frequently  grouped  together  as  ganoids.  All  these 
agree  in  a  number  of  particulars  of  considerable  importance. 


TELEOSTOMOUS  FISHES.  243 

The  gill  slits  do  not  open  directly  to  the  external  world,  but 
into  a  gill  chamber  formed  by  an  operculum  or  fold,  which 
extends  backwards  on  either  side  from  the  hyoid  arch  ;  and  this 
operculum  is  strengthened  by  bone.  The  body  may  be  naked 
or  scaled,  but  placoid  scales  never  occur,  and  claspers  are  never 
developed  in  connection  with  the  pelvic  fins. 

The  most  marked  characters  are  presented  by  the  skeleton. 
In  this  the  cartilage  may  be  partially  or  almost  completely 
replaced  by  bone,  and  besides,  there  is  always  an  extensive 
formation  of  membrane  bones,  unknown  in  the  recent  members 
of  the  other  groups  of  fishes.  In  all  except  the  sturgeons  the 
vertebral  centra  are  ossified,  and  in  all  except  the  sturgeons  and 
the  garpikes  the  vertebrae  are  amphiccelous.  More  or  less 


SOr 


FIG.  245.  Skull  of  pike,  Esox  lucius,  from  Huxley.  An,  angulare;  Av, 
articulare ;  Brg,  branchiostegals ;  D,  dentary ;  HM,  hyomandibular ;  IOp,  inter- 
opercle;  Alt,  metapterygoid ;  MX,  maxillary;  Op,  operculum;  PI,  palatine;  Prnxt 
premaxilla;  Prf,  prefrontal  ;  PrOp,  preoperculum  ;  Qu,  quadrate;  SOp,  subopercle; 
SOr,  suborbital ;  Sy,  symplectic. 

extensive  ossifications  occur  in  the  chondrocranium,  and  besides, 
a  large  number  of  dermal  bones  are  developed,  which  roof  in 
the  cranium  above  and  build  it  out  in  other  places.  Excepting 
the  dipnoi,  the  cranial  structures  of  which  will  be  described 
later,  the  most  constant  and  most  characteristic  of  these  bones 
are  the  following  :  The  upper  jaw  is  formed  by  a  pair  each  of 
maxillaries  and  premaxillaries,  while  the  roof  of  the  mouth  is 
formed  by  a  pair  each  of  vomers  and  palatines  and  a  parasphe- 
noid,  all  of  which  may  bear  teeth.  Thus  the  pterygoquadrate 
no  longer  forms  the  upper  jaw  as  in  the  lower  groups,  but  be- 
comes deeper  in  position,  and  undergoes  more  or  less  extensive 
ossification,  sometimes  developing  but  two  bones,  —  pterygoid 


244  CLASSIFICATION  OF   VERTEBRATES. 

and  quadrate ;  or  again,  the  pterygoid  may  be  differentiated 
into  several  elements  (ento-,  ecto-,  meso-,  and  metapterygoid)  ; 
but  in  all  cases  the  quadrate  furnishes  the  support  for  the  lower 
jaw.  The  quadrate,  in  turn,  is  attached  to  the  skull  by  a  sus- 
pensor  apparatus  formed  of  the  hyomandibular  alone  ;  or  a 
membrane  bone,  the  sympletic,  may  intervene  between  quadrate 
and  hyomandibular.  Thus  this  skull  is  hyostylic.  The  prim- 
itive lower  jaw  (Meckel's  cartilage)  becomes  incased  in  mem- 
brane  bones,  of  which  a  pair  of  dentaries  in  front  are  always 
present,  while  an  articulare  and  an  angulare  may  be  added  on 
either  side.  The  roofing  bones  of  the  cranium  may  be  a  pair 
each  of  parietals  (small),  frontals,  and  nasals.  Beneath  these  is 
a  large  fontanelle  in  the  chondrocranium.  In  the  chondrocra- 
nium  itself  the  following  ossifications  may  occur  :  At  the  base 
of  the  skull  the  four  occipitals  (basi-,  ex-,  and  supraoccipital)  ; 
in  the  ear  region  five  otic  bones,  the  sphen-,  pter-,  and  epiotic 
above,  the  pro-  and  opisthotic  below.  Occasionally  the  opis- 
thotic  may  be  absent.  Basi-  and  presphenoid  never  occur,  their 
place  being  supplied  by  the  parasphenoid.  Ali-  and  orbito- 
sphenoid  are  sometimes  well  developed,  sometimes  inconspicu- 
ous or  absent,  while  the  ethmoid  region  bears  three  ossifications, 
-  a  mesethmoid,  and  a  pair  of  ectethmoids. 

The  operculum  is  supported  by  the  hyoid  arch,  the  lower 
portion  of  which,  the  hyoid  proper,  is  connected  with  the  hyo- 
mandibular by  a  small  interhyal  bone.  The  operculum  consists 
of  two  portions,  —  a  dorsal,  containing  several  flattened  bones  : 
above,  a  preoperculum  and  an  operculum  proper ;  below  and 
behind,  a  suboperculum  and  an  interoperculum,  all  of  which 
may  be  regarded  as  modified  branchiostegals  of  the  hyoman- 
dibular ;  the  ventral  part  of  the  operculum  is  supported  by  the 
long  and  slender  branchiostegals  of  the  hyoid.  The  branchial 
arches  are  united  below  by  a  copula  composed  of  the  fused  basi- 
branchials,  which  extends  forward,  connecting  the  series  with 
the  hyoid.  Frequently  the  fifth  arch  is  modified  into  a  tooth- 
bearing  accessory  jaw,  the  so-called  pharyngeal  bones. 

The  shoulder  girdle  is  well  developed.  In  the  cartilaginous 
arch  two  ossifications  —  scapula  and  coracoid  —  appear  on  either 
side,  and  besides  these  a  large  membrane  bone,  the  cleithrum 


TELEOSTOMOUS  FISHES. 


245 


(clavicle  of  authors),  the  cleithra  of  the  two  sides  frequently 
uniting  below.  Other  smaller  membrane  bones,  some  of  them 
less  constant,  are  the  supra-,  post-,  and  infraclavicles  ;  and  the 
arch  is  usually  articulated  to  the  otic  region  of  the  skull,  either 
by  the  supraclavicle,  or  by  the  intervention  of  a  post-temporal 
bone  between  the  supraclavicle  and  the  epiotic.  The  pelvic 
arch  is  either  greatly  reduced  (ganoids)  or  entirely  absent 
(teleosts),  its  place  being  supplied  by  the  greatly  enlarged 


FIG.  246.  Skull  of  perch.  B,  basalia  of  fin ;  C,  coracoid ;  CL,  cleithrum  ;  7(9, 
infraopercular  ;  J/,  maxillary  ;  O,  opercular  ;  PC,  postclavicula  ;  PO,  preopercular  ; 
PMy  premaxilla;  PT>  post-temporal;  Q,  quadrate;  fi,  radialia  of  fin;  S O,  sub- 
ocular  chain  of  bones,  also  suboperculum ;  S,  scapula. 

basalia  (see,  however,  p.  173).  The  dorsal  and  anal  fins  are  sup- 
ported upon  small  interspinous  bones  embedded  in  the  flesh  and 
usually  alternating  with  the  neural  and  haemal  spines,  or  they 
may  be  more  numerous  than  these.  The  caudal  fin  is  either 
heterocercal,  or  the  lower  lobe  may  be  so  well  developed  that 
the  homocercal  condition  occurs  (p.  229).  The  ribs,  except  in 
Polypterus,  where  both  types  occur  (p.  145),  are  modified  haemal 
arches ;  and  the  flesh  is  further  supported  by  intermuscular 


246 


CLASSIFICATION  OR    VERTEBRATES. 


bones,  called  epineurals,  epicentrals,  or  epipleurals,  accordingly 
as  they  are  articulated  to  neural  arch,  centrum,  or  ribs. 

The  mouth  is  usually  armed  with  teeth  ;  and  these  may  occur 
not  only  on  the  bones  which  form  the  edge  of  the  jaws  (pre- 
maxillary,  maxillary,  dentary),  but  on  those  which  form  the  roof 
of  the  mouth  (palatine,  vomer,  pterygoid),  and  also  on  the  pha- 
ryngeal  bones.  The  alimentary  canal  usually  has  the  regions 

well  differentiated,  and  in 
the  ganoids  a  spiral  valve 
occurs  in  the  intestine. 
Pyloric  caeca,  from  a  sin- 
gle one  to  two  hundred 
in  number,  are  common. 
There  is  no  cloaca,  as  the 
urogenital  ducts  always 
open  behind  the  vent. 

A  spiracle  occurs  only 
in  some  ganoids.  The 
gills  differ  from  those  of 
the  lower  fishes  in  the 
reduction  of  the  inter- 
branchial  septum,  so  that 
the  gills  themselves  pro- 
ject beyond  the  arch  into 
the  opercular  cavity  like 
the  teeth  of  a  comb.  In 
the  lophobranchs  the  gill 
filaments  are  replaced  by 
curious  tufts.  An  air- 
bladder  or  swim-bladder  is  usually  present.  It  arises  as  an 
outgrowth  from  the  dorsal  side  of  the  oesophagus  (except  in 
Polypterus),  which  soon  becomes  differentiated  into  bladder 
and  duct.  In  the  lower  forms  the  duct  remains  open  through- 
out life  (ganoids  and  physostomi),  but  in  the  physoclisti  it 
closes  later,  and  the  bladder  loses  all  connection  with  the 
exterior.  In  many  ganoids  and  some  teleosts  the  inner  sur- 
face becomes  plicated.  In  most  forms  it  receives  its  blood- 
supply  from  the  aorta  direct,  or  by  way  of  the  coeliac  axis  ;  but 


FIG.  247.  Relations  of  gill  clefts  in  a 
teleost.  O,  operculum,  enclosing  a  branchial 
chamber. 


TELEOSTOMOUS  FISHES.  247 

in  Polypterus  it  comes  from  the  radix  aortse,  and  therefore  it 
receives  only  arterial  blood.  The  bladder  serves  as  a  hydro- 
static apparatus,  but  there  is  also  evidence  to  show  that  at  least 
in  some  fishes  it  is  to  some  extent  respiratory  as  well.  For  the 
relations  of  the  bladder  to  the  ear,  see  p.  255. 

The  brain  is  noticeable  for  the  large  size  of  the  optic  lobes 
and  the  cerebellum.  The  cerebrum  is  rudimentary,  and  consists 
of  merely  corpora  striata  and  a  non-nervous  pallium  in  the  tele- 
osts,  but  in  the  ganoids  larger  hemispheres  occur.  In  the 
ganoids  there  is  a  true  optic  chiasma,  but  in  the  teleosts  the 
optic  nerves  cross  (p.  61).  The  twixt  brain  is  short.  The  olfac- 
tory lobes  in  most  teleosts  and  in  the  ganoids  are  joined  to 
the  cerebrum  ;  but  in  a  few  forms  a  long  olfactory  tract  inter- 
venes. 

The  urogenital  organs  of  the  teleostomes  will  repay  further 
study,  for  there  are  many  points  as  yet  in  doubt.  The  perma- 
nent excretory  organ  is  the  mesonephros  ;  only  in  Fierasfer  and 
Dactylopterus  does  the  pronephros  retain  its  excretory  functions. 
In  all  others,  while  it  may  be  of  large  size,  it  degenerates  into 
a  lymphatic  or  adenoid  structure.  The  pronephric  ducts  never 
divide  into  Mtillerian  and  Wolffian  ducts,  but  serve  solely  as 
ureters.  Usually  the  two  unite  behind  and  form  a  urinary 
bladder  of  some  size,  the  common  opening  being,  except  in 
a  few  teleosts,  behind  the  vent.1  The  usually  paired  gonads 
vary  in  the  way  in  which  their  products  reach  the  exterior.  In 
the  female  salmonids  and  eels,  the  eggs  are  discharged  directly 
into  the  coelom,  from  which  they  escape  into  a  urogenital  sinus 
by  means  of  a  pair  of  slit-like  openings,  often  called  pori  abdom- 
inales,  but  which  are  apparently  not  homologous  with  the  sim- 
ilarly named  openings  in  the  elasmobranchs.  In  most  ganoids 
and  in  a  few  teleosts,  two  longitudinal  folds  arise  in  the  peri- 
toneum, the  edges  of  which  unite  so  that  a  tube,  the  oviduct, 
results,  which  opens  freely  into  the  ccelom.  In  most  teleosts, 
however,  these  folds  are  continued  to  the  ovary,  so  that  the 
eggs  do  not  pass  into  the  general  body  cavity,  but  fall  at  once 
into  these  tubes,  the  lumen  of  which  is,  as  is  readily  seen,  a  part 

1  In  Pediculati  and  some  symbranchs  and  plectognaths  the  urinary  opening  is  in  the 
hinder  end  of  the  intestine. 


248 


CLASSIFICATION  OF   VERTEBRATES. 


of  the  coelom.  In  the  males  there  is  either  a  simple  tube  which 
connects  each  testis  with  the  urogenital  sinus,  or  there  may 
intervene  between  tube  and  testis  a  system  of  smaller  canals, 
the  vasa  efferentia. 

Legion  I.     Ganoidea. 

Teleostomes  in  which  the  body  is  either  naked  or  covered 
with  ganoid  or  cycloid  scales,  or  bears  bony  plates  ;  the  skeleton 
either  largely  cartilaginous  or  well  ossified  ;  the  tail  diphy-,  hetero- 
or  homocercal ;  ventral  fins  always  abdominal  in  position  ;  fulcra 
present  in  most  recent  and  in  fossil  forms  ;  swim-bladder  with 
duct ;  intestine  with  a  spiral  valve  ;  an  optic  chiasma  present ; 
heart  with  a  conus  arteriosus  ;  eggs  with  a  total  segmentation. 

The  group  of  ganoids  contains  but  a  few  recent  forms,  the 
remnants  of  a  much  larger  fauna  in  past  times.  Its  members 
are  widely  distributed  over  the  globe,  North  America,  however, 
having  the  greater  proportion  of  the  species,  most  of  which  are 
inhabitants  of  fresh  water.  In  the  definition  above  fulcra  are 
mentioned.  These  are  spine-like  scales  upon  the  anterior  sur- 
faces of  the  fins. 

So  far  as  they  have  been  studied,  the  eggs  of  the  ganoids 
undergo  a  total  segmentation  ;  but,  owing  to  the  presence  of  a 


FIG.  248.  Segmenting  egg 
of  Amia,  showing  the  unequal 
cleavage,  after  Dean. 


FIG.  249.  Larva  of  Amia,  about  the  time  of 
hatching,  showing  the  sucking  disk  at  the  tip  of 
the  snout,  after  Dean. 


large  amount  of  yolk,  the  resulting  cells  are  very  unequal  in 
size,  the  large  cells  being  at  one  pole  of  the  egg,  the  smaller  at 
the  other.  In  the  sturgeon  the  central  nervous  system  develops 
as  a  tube  ;  but  in  the  other  forms  it  is  at  first  a  solid  keel,  and 
only  later  does  a  lumen  appear  by  splitting.  The  larvae  of 


GANOIDS.  249, 

Amia  and  Lepidosteus  are  noticeable  for  the  sucking  disk  de- 
veloped on  the  front  of  the  head  ;  the  larva  of  the  sturgeon  has 
balancer-like  structures  between  the  mouth  and  nose. 

ORDER   I.     CROSSOPTERYGII. 

Ganoids  with  diphy-  or  heterocercal  tails  ;  pectoral  fins  with 
a  large  basal  portion  covered  with  scales,  the  ventral  fins  usually 
much  like  the  pectorals,  the  former  abdominal  in  position  ;  body 
covered  with  rhombic  or  circular  ganoid  scales  ;  a  pair  of  gular 
(or  'jugal')  plates  in  place  of  branchiostegals ;  no  fulcra;  dor- 
sal fins  two,  or  a  single  one  divided  into  many  finlets.  The 
crossopterygians  are  largely  extinct,  but  two  genera  persisting 
to-day.  The  group  first  appears  in  the  Devonian. 


FIG.  250.     Diplurus  longicaudatus,  from  Dean.     A,  position  of  the 
calcified  air-bladder. 

The  CCELACANTHID^E  (AcTixiSTiA)  have  unossified  centra  and  cycloicF 
scales,  two  dorsal  fins,  diphycercal  caudal,  and  ossified  swim-bladder.. 
Ccelacanthus,  carboniferous  of  Europe  and  Ohio ;  Diplurus,  trias  of  New 
Jersey.  CYCLODIPTERINI  (RHIPIDISTIA)  have  the  vertebrae  partially  ossi- 


FIG.  251.     Head  of  larval  Polypterus,  after  Steindachner,  from  Dean. 
EG,  external  gill. 

fied ;  tail  heterocercal ;  scales  enamelled  and  rounded  behind ;  a  third' 
gular  plate  between  the  other  two.  Holoptychius,  Devonian  of  Europe  and 
America;  Onychodus  and  Etisthenopteron,  Devonian  of  America.  RHOM- 


250  CLASSIFICATION  OF   VERTEBRATES. 

BODIPTERINI,  with  two  dorsals,  partially  ossified  vertebral  centra  ;  diphy-or 
heterocercal  tail;  two  large  and  several  smaller  gular  plates.  Osteolepis, 
Devonian,  Europe.  The  POLYPTERID^:  (CLADISTIA)  with  two  living 
genera  (Polypterus  from  the  Nile,  Calamoichthys,  greatly  elongate  and 
lacking  ventral  fins,  from  Old  Calabar)  are  most  closely  allied  to  the  last. 
The  vertebrae  are  ossified,  the  caudal  diphycercal ;  the  dorsal  fin  elongate 
and  divided  into  finlets ;  pectorals  with  a  short,  scaled  basal  axis ;  the  body 
covered  with  rhomboid  ganoid  scales.  No  fulcra  exist.  In  the  skull  epi- 
and  opisthotics  are  not  distinct ;  there  are  two  sphenoidals  and  ecteth- 
moids  ;  the  parietals  and  frontals  are  paired,  and  the  gular  plate  is  double. 

ORDER    II.     CHONDROSTEI. 

In  the  sturgeons  and  paddle-fish  there  is  but  slight  ossifica- 
tion of  the  cartilage,  the  vertebral  centra  being  unossified,  while 
in  the  chondrocranium  only  otic  and  ectethmoid  ossifications 
appear.  The  skull  is  covered  with  membrane  bones,  the  parie- 
tals and  frontals  being  paired,  while  the  large  parasphenoid 
extends  back  beneath  the  anterior  vertebrae.  A  premaxilla  is 
absent,  and  only  a  dentary  is  present  in  the  lower  jaw.  The 
jaw  itself  is  suspended  by  sympletic  and  hyomandibular  carti- 
lages, both  partially  ossified,  the  mouth  itself  being  ventral  as 
in  elasmobranchs.  The  operculum  is  large,  but  its  elements  are 
poorly  developed ;  the  branchiostegals  are  weak  or  wanting. 
The  body  is  either  naked  or  covered  with  rows  of  bony  plates, 
'which  are  continued  upon  the  upper  lobe  of  the  heterocercal  tail, 
upon  which  fulcra  are  also  strongly  developed.  The  ventral 
fins  have  a  row  of  cartilaginous  basalia. 

Two  recent  families  are  recognized.  In  the  ACIPENSERID^E  or  sturgeons 
the  body  is  covered  with  five  rows  of  keeled  bony  plates,  the  skin  between 
the  rows  bearing  small  granules.  The  mouth  is  toothless  in  the  adult,  and 


FlG.  252.     Common  sturgeon,  Acipenser  stuno^  after  Goode. 

In  front  of  it  are  four  barbels.  The  gill  slits  are  four  in  number,  and  the 
operculum,  which  does  not  completely  cover  the  slits,  bears  an  accessory 
gill.  The  air-bladder  is  large  and  simple,  and  the  stomach  has  pyloric 


GANOIDS.  251 

appendages.  The  dorsal  and  anal  fins  are  posterior  in  position,  the  anal 
being  anterior  to  the  dorsal.  In  Acipenser  there  is  a  spiracle,  and  the 
naked  skin  between  the  plates  extends  to  the  tail.  About  twenty  living 
species  are  known,  half  a  dozen  from  North  America.  From  the  ovaries 
caviare  is  made,  while  the  air-bladders  furnish  isinglass.  The  genus 
appear  in  the  London  clay  (eocene).  The  shovel-nose  sturgeons  (Scaphi- 
rhynchus},  one  species  of  which  is  American,  lack  the  spiracle,  have  the 
plates  forming  a  complete  armor  on  the  depressed  tail,  while  the  caudal  fin 
ends  in  a  filament.  In  the  POLYODONTID^E  (SELACHOSTOMI),  represented 
to-day  by  Polyodon  spathula  in  the  U.S.,  and  Psephurus  in  China,  the  skin 
is  smooth,  the  snout  is  prolonged  into  a  long  blade  (whence  the  name  paddle- 
fish),  the  maxillary  is  obsolete,  a  spiracle  (lacking  pseudobranchs)  occurs. 
The  PAL/EONISCID^,  which  range  from  the  Devonian  to  the  lias,  have 
small  conical  or  styliform  teeth,  simple  dorsal  and  heterocercal  tail,  and 
rhombic  scales.  Palceoniscus,  Europe,  U.  S. ;  Eurylepis,  U.  S.  Allied 
is  Platysomus  from  the  carboniferous  of  Europe  and  Illinois.1 

ORDER   III.     HOLOSTEI. 

Ganoids  with  well-ossified  skeletons  ;  tail  heterocercal ;  body 
with  ganoid  or  cycloid  scales  ;  fulcra  frequently  present  ;  branch- 
iostegals  and  operculum  well  developed,  and  frequently  a  median 
gular  plate ;  mouth  terminal,  with  teeth  ;  fins  without  a  scaled 
basal  region  ;  the  ventrals  with  the  proximal  skeletal  elements 
reduced,  much  as  in  teleosts. 

The  garpikes,  LEPIDOSTEID^E  (GiNGLYMODi)  are  closely  related,  struc- 
turally, to  the  palaeoniscid  forms  of  the  chondrostei.  They  have  opistho- 
coelous  vertebrae,  the  body  covered  with  rhombic  scales,  greatly  elongate 


FIG.  253.     Garpike,  Lepidosteus  osseus,  from  Tenney. 

jaws,  these,  like  the  vomers  and  palatines,  with  sharp  teeth  ;  and  numerous 
pyloric  caeca.  The  living  species  of  the  only  existing  genus  Lepidosteus 
are  all  American.  The  common  garpike,  L.  osseus,  is  widely  distributed ; 

1  The  group  ACANTHODID/E,  which  combines  ganoid  and  elasmobranch  characters, 
may  be  mentioned  here.  The  cartilaginous  skeleton,  spine  in  front  of  the  dorsal,  absence  of 
opercular  elements,  are  elasmobranch  characters,  while  the  presence  of  spines  in  the  pectorals, 
and  especially  of  bones  in  the  orbital  region  and  in  the  roof  of  the  cranium,  and  the  absence 
of  claspers,  recall  the  teleostomes.  These  forms  occur  in  paleozoic  rocks.  Acanthodes  occurs 
in  U.  S. 


252 


CLASSIFICATION  OF   VERTEBRATES. 


the  alligator  gar,  L.  tristaechus  of  the  southern  states,  reaches  a  length  of 
ten  feet.  Allied  fossil  forms  are  numerous,  Catopterus  being  represented 
in  the  triassic  rocks  of  the  Connecticut  valley.  Lepidotus  ranges  from 
the  trias  to  the  Jura  of  Europe.  Aspidorhynchus  had  a  snout  something 
like  that  of  the  sword-fish.  In  the  AMIID^E  (HALECOMORPHI)  the  .vertebrae 
are  amphicoelous,  the  scales  cycloid,  teeth  on  pterygoids  as  well  as  on 
vomers  and  palatines,  no  pyloric  caeca.  Amia  calva,  the  bow  fin  of  the 
eastern  U.  S.,  is  the  only  living  species.  The  genus  dates  from  the  eocene. 
Allied  fossil  forms  are  Eurycormus,  Callopterus,  Caturus,  and  Pachycor- 
mus,  ranging  from  the  lias  to  the  Jurassic. 


Legion  II.     Teleostei. 

Fishes  with  the  bony  skeleton  well  developed,  the  cranium 
and  the  vertebral  centra  ossified,  the  latter  amphicoelous ;  tail 
diphy-  or  homocercal ;  spiral  valve  and  conus  arteriosus  not 
developed ;  no  optic  chiasma ;  scales, 
when  present,  cycloid  or  ctenoid. 

The  group  of  teleosts  or  bony  fishes 
so  closely  follows  the  ganoids  that  some 
students  do  not  distinguish  between 
them.  There  are,  however,  some  dis- 
tinctions between  the  two  groups,  while 
the  matter  of  convenience  warrants 
their  recognition. 

In  a  few  teleosts  the  skin  is  naked 
or  covered  with  bony  plates,  but  usually 
the  body  is  covered  with  scales  of  the 
cycloid  or  ctenoid  type.  In  a  few  the 
tail  is  diphycercal,  but  usually  it  is  hom- 
ocercal. The  fulcra,  so  characteristic  of 
most  ganoids,  never  occur.  The  skele- 
ton is  well  ossified,  this  being  especially 
true  of  the  skull,  where  the  cartilages 
are  almost  entirely  replaced  by  bone. 
The  operculum  and  its  skeleton  are 
well  developed,  branchiostegals  are  pres- 
ent, and  gular  plates  rarely  occur.  The 
paired  fins  never  have  a  basal  lobe  ;  and  the  ventrals,  when  pres- 
ent, may  either  be  near  the  vent  or  far  forward,  beneath  the 


FlG.  254.  Breathing 
valves  of  teleosts,  after 
Dahlgren.  av^  anterior  or 
oral  valves  open;  g;  gills; 
o,  oesophagus ;  pv,  posterior 
valves.  At  expiration  the 
anterior  valves  close,  the 
posterior  open  ;  the  enlarge- 
ment and  contraction  of  the 
oral  cavity  being  brought 
about  by  motion  of  the  oral 
walls  (black). 


TELE OS TS. 


253 


throat.  A  spiral  valve  is  absent  except  in  the  single  genus 
Cheirocentnis,  while  the  conus  arteriosus  is  represented  only  in 
the  genus  Butrinus  (see,  however,  p.  227).  The  bulbus  aortae 
is  large.  The  pallium  of  the  cerebrum  is  non-nervous  in  char- 
acter (Fig.  53),  and  the  optic  nerves  cross  (Fig.  63)  and  never 
unite  in  a  chiasma. 

An  interesting  discovery  has  recently  been  made,  that  in 
many  if  not  in  all  teleosts  breathing-valves  exist,  one  pair  at 


FIG.  255*  Five  stages  in  the  development  of  the  cunner  (^Ctenolabrus).  A, 
two  cells,  resting  upon  the  yolk  ;  Z?,  surface  view  of  the  eight-celled  stage ;  C,  the 
blastoderm  covers  about  one-third  of  the  yolk,  the  segmentation  cavity  (Y)  showing 
through;  the  embryo  (<?)  is  outlined,  while  the  blastoderm  is  margined  by  a  thicker 
rim  (r)  of  inturned  entoderm ;  D,  the  blastoderm  has  covered  three-quarters  of 
the  yolk;  the  three  primary  brain  regions  are  differentiated,  and  myotomes  have 
appeared ;  £,  a  stage  shortly  before  hatching,  the  yolk  having  been  largely  absorbed, 
and  the  tail  having  grown  out. 

the  mouth,  the  other  behind  the  gills,  so  arranged  that  when  the 
mouth  cavity  is  enlarged  water  can  only  flow  in  through  the 
mouth  ;  when  contracted,  it  can  only  escape  through  the  gills. 
The  eggs  of  the  teleosts  are  peculiar  in  the  almost  complete 
separation  between  yolk  and  protoplasmic  portions  ;  the  latter 
alone  dividing  in  the  early  stages  of  development,  and  giving 


254  CLASSIFICATION  OF   VERTEBRATES. 

rise  to  the  blastoderm,  which  gradually  extends  over  the  yolk 
A  peculiarity  in  development  is  the  formation  of  the  central 
nervous  system  as  a  solid  cord  or  « keel,'  in  which  a  lumen 
appears  later  by  splitting.  In  many  teleosts  a  peculiar  saccular 
structure,  known  as  Kupffer's  vesicle,  occurs  near  the  hinder 
end  of  the  alimentary  tract.  It  disappears  before  hatching,  and 
its  significance  is  not  yet  understood. 

The  teleosts  first  appear  in  the  triassic,  and  in  the  creta- 
ceous they  exceed  the  ganoids  in  number,  while  to-day  the 
group  includes  the  vast  majority  of  the  forms — some  fifteen 
thousand  in  all — commonly  known  as  fishes. 

The  classification  of  the  group  is  in  a  very  unsatisfactory 
condition,  especially  as  regards  the  vast  order  acanthopteri, 
where  as  yet  it  is  almost  impossible  to  frame  sub-orders,  owing 
to  the  extent  to  which  the  different  families  intergrade,  and  the 
limited  degree  to  which  we  can  ascertain  the  lines  of  descent. 
The  trouble  is  largely  based  upon  the  fact  that  many  lines  have 
persisted,  so  that  our  artificial  systems  cannot  easily  be  applied. 
The  teleosts  have  apparently  had  two  lines  of  descent,  one 
leading  from  sturgeon-like  ancestors  to  the  Ostariophysi  (for 
it  is  difficult  to  believe  that  the  peculiar  Weberian  apparatus 
has  been  evolved  twice  in  the  history  of  the  fishes),  and  the 
other  from  some  Amta-like  form  to  the  isospondyli,  and  these 
to  the  apodes  on  the  one  hand,  and  to  the  acanthopteri  and  other 
orders.  For  convenience  the  division  Physostomi  is  retained 
for  all  those  fishes,  the  ostariophysi  excepted,  in  which  the  duct 
of  the  air-bladder  remains  open  permanently  ;  the  term  Physo- 
clisti  is  frequently  employed  for  the  remaining  groups  in  which 
the  duct  is  closed,  as  well  as  for  those  in  which  the  bladder 
itself  has  disappeared. 

ORDER   I.     OSTARIOPHYSI. 

Teleosts  with  the  anterior  vertebrae  modified  into  a  '  Web- 
erian apparatus  '  connecting  the  inner  ear  with  the  large  swim- 
bladder.  Fins  without  spiny  rays,  or  at  most  with  a  single 
spine  in  front  of  pectorals  and  dorsals  ;  ventrals  abdominal  in 
position ;  duct  to  air-bladder  persisting. 


TELEOSTS.  255 

(upwards  of  1,000  species),  skin  without  scales,  naked,  or 
covered  with  bony  plates;  premaxilla  toothed  and  forming  upper  jaw; 
maxilla  with  barbels.  Mostly  fresh-water  forms  from  South  America  and 
Africa.  A miurus  contains  our  smaller  cat-fish  (bull-heads  or  horned  pout); 
closely  allied  is  the  nearly  blind  Gronias  from  caves  in  Pennsylvania. 
Ictahtrus,  larger  river  cat-fish.  Loricaria  and  Clarias  are  armored ;  Mal- 
apterurus  of  Africa  is  electric.  Fossil  siluroids  appear  in  the  eocene  of 
Wyoming. 


FIG.  256.  Weberian  apparatus  of  the  carp,  Cyprinus,  after  Weber,  k,  sinus 
impar  of  ear;  r,  pneumatic  duct ;  s,  utriculus  of  ear;  /,  spinous  processes  of  ante- 
rior vertebras;  .r,  Weberian  chain,  leading  to  2,  the  air-bladder. 

CYPRINID^E,  narrow  mouths,  frequently  with  barbels ;  the  jaws  tooth- 
less, but  with  teeth  on  the  pharyngeals,  scales  cycloid,  fins  without  spines. 
About  1,000  species  in  fresh  water.  Cyprinus,  the  carp,  and  Carassius 
the  gold-fish,  come  from  China.  Leucisus  and  Nolropis,  numerous. 
Ptvchochriius  of  the  Pacific  coast,  4  feet  long.  Barbus  Cobitis.  The 
Cyprindae  appear  in  eocene.  The  suckers  (CATOSTOMID^;)  have  sucker- 
like  toothless  mouths.  About  60  species,  mostly  from  North  America, 
Ictobius,  Catostomus .  Amyzon,  from  American  eocene.  The  CHARA- 
CINID^:  (Erythrinns,  Ckaracinns)  are  tropical. 

The  GYMNONOTI  of  South  America  are  eel-like,  the  dorsal  fin  reduced 
or  absent.  Gymnotus  electricus,  the  electric  eel.  MORMYRID^E,  fresh- 
water, Africa. 

ORDER    II.     PHYSOSTOMI. 

A  duct  to  the  air-bladder,  fins  without  spines,  ventral  fins 
abdominal  in  position  ;  no  Weberian  apparatus  ;  gills  comb-like ; 
scales,  when  present,  usually  cycloid 

SUB-ORDER   i.     ISOSPONDYLI. 

Mostly  marine  fishes  with  distinct  opercular  bones ;  shoulder  girdle 
connected  with  the  cranium  by  a  post-temporal ;  maxillary  and  premaxillary 
distinct,  no  barbels ;  ventral  fins  sometimes  wanting,  pharyngeal  bones 
simple. 


2$6  CLASSIFICATION  OF   VERTEBRATES. 

CLUPEID/E  (herrings),  head  naked  ;  lateral  line  lacking ;  no  adipose  dor- 
rsal ;  weak  teeth  or  none.  The  species  number  about  125,  but  are  very 
numerous  as  individuals.  Clupea  (dates  from  the  cretaceous),  shad,  ale- 
wife,  and  herring,  Brevoortia,  the  menhaden.  Allied  wzAlbulia,  Dorosoma, 


FlG.  257.     Herring,  Clupea  harengus. 

and  Engraulis.  Arapaima  from  South  America  reaches  a  length  of  15 
feet.  The  ancestral  Leptolepis  from  the  Jurassic  shows  marked  ganoid 
features.  ALEPOCEPHALUXE,  deep-sea  forms.  SAUROCEPHALHXE,  extinct; 
Xiphactinus  (Portheus]  from  American  cretaceous. 

The  SALMONID^E,  containing  the  salmon,  trout,  etc.,  are  among  the 
most  valuable  of  food  fishes.  They  have  an  adipose  fin ;  teeth  variable ; 
lateral  line  present ;  no  oviducts,  the  eggs  passing  out  by  the  pori  ab- 
dominales.  Coregonus,  the  white  fishes ;  Salmo,  the  salmon  and  trout  (the 
Pacific  salmon  sometimes  placed  in  sub-genus  Oncorhynchus).  Closely 


FIG.  258.     Atlantic  salmon,  Salmo  salar,  after  Goode. 


allied  is  the  smelt,  Osmerus.     The  fossils   not   readily  distinguished   from 
the  Clupeidae. 

A  number  of  forms,  mostly  from  the  deep  sea,  are  grouped  as  INIOMI, 
many  of  which  are  furnished  with  phosphorescent  organs.  In  Ipnops  the 
eyes  are  absent,  but  the  head  is  covered  with  a  luminous  plate.  Scopelus 
Chauliodus.  Scopelid-like  forms  occur  in  the  cretaceous. 

SUB-ORDER  2.     APODES. 

Degenerate  eel-like  physostomi  without  ventral  fins,  mouth  and  oper- 
culum  reduced,  scales  minute  or  lacking;  scapular  arch  free  from  the  skull. 
The  true  eels  (  ANGUILLID^:)  have  the  gill  openings  well  developed ;  pectoral 


TELE  OS  TS.  257 

fins  present.  Anguilla  vulgaris,  the  common  eel  of  Europe  and  America. 
Conger  vnlgaris,  conger  eel,  almost  cosmopolitan.  The  name  Leptoceph- 
alus  was  given  to  the  young  of  both  Anguilla  and  Conger.  MUR/ENID^:, 
gill  slits  small  and  rounded,  pectorals  lacking.  Mjir&na,  the  murrys  of 
warmer  seas.  The  Apodes  are  an  old  group,  Anguilla  dating  from  the 
cretaceous.  They  have  possibly  descended  from  the  Isospondyli. 

SUB-ORDER  3.     LYOMERI. 

Deep  sea  physostomes  with  eel-shaped  bodies,  large  heads,  5-6  simple 
gill  arches  ;  skull  imperfectly  ossified.  In  some  features  they  appear  de- 
generate, in  some  primitive.  But  few  specimens  known.  Eurypharynx, 
Gastrostomus. 

SUB-ORDER  4.     HAPLOMI. 

Physostomes  with  the  head  usually  scaly,  scales  cycloid;  shoulder 
girdle  attached  to  the  cranium  ;  teeth  present  ;  no  adipose  dorsal.  UMBRID^E 
(mud-minnows),  teeth  villiform;  maxillaries  forming  the  lateral  part  of  the 
jaw.  Umbra.  ESOCID^:,  pikes,  maxilla  similar  to  the  last  ;  teeth  are  card- 
like  and  unequal.  Esox,  pickerel,  pike,  muskalunge.  Esox  dates  from  the 
miocene,  Ischyrhiza  from  the  cretaceous  of  the  United  States.  CYPRINO- 
DONTiDyE,  premaxilla  forms  the  entire  upper  jaw;  vent  normal.  Fundulus, 
the  mummichogs  or  killifishes,  Anableps.  Cyprinodon.  Lebias  from  the 
miocene.  AMBLYOPSIDJE  (Heteropygii)  ;  jaws  as  in  cyprinodonts  ;  vent  in 
the  branchial  region  ;  species  mostly  cave  inhabitants  (American)  and 
have  greatly  degenerate  eyes.  Amblyopsis,  Chologaster.  The  STRA— 
are  extinct.  Ciwolichthys,  upper  cretaceous. 


ORDER   III.     SYNENTOGNATHI. 

Large  physoclistous  bladder  ;  fins  without  spines  ;  lower 
pharyngeals  united  into  a  single  bone  ;  shoulder  girdle  con- 
nected to  cranium  ;  a  row  of  keeled  scales  on  the  belly  ;  marine. 
These  forms  are  allied  to  the  percesoces  and  the  spine-finned 
fishes. 

ExocGETiD,E  or  ScOMBRESOCiD/E,  gill  openings  wide;  jaws  more  or 
less  prolonged  ;  maxillary  and  premaxillary  free  ;  and  the  third  upper  pha- 
ryngeal  greatly  enlarged.  Scomberesox,  bill-fish,  Exoccetus,  flying-fish. 
Isteus,  etc.,  cretaceous.  BELONID^:,  jaws  greatly  elongate,  the  lower  the 
longer  ;  maxillaries  and  premaxillaries  closely  united,  third  upper  pha- 
ryngeal  not  enlarged.  Tylosurus,  needle-fish  ;  Belone,  bony  gars,  appears 
in  the  miocene. 

ORDER    IV.     HEMIBRANCHII. 

Teleosts  with  the  pharyngeals  reduced  in  number,  the  lower 
not  united  ;  gills  pectinate  ;  mouth  bounded  by  premaxilla  above  ; 
scapular  arch  connected  to  cranium  ;  ventrals  sub-abdominal. 


258 


CLASSIFICATION  OF   VERTEBRATES. 


GASTEROSTEID^E  ;  body  naked  or  with  bony  plates  ;  dorsal  with  spines  ; 
teeth  sharp,  in  jaws  alone ;  anal  with  one  spine ;  fresh  and  brackish 
water.  Gasterosteus,  Apeltes,  sticklebacks.  CENTRISCID^E,  two  dorsals,  the 
first  spiny;  marine.  FISTULARIID/E,  bony  plates  are  present;  the  dorsal  is 
without  spines  ;  snout  produced  into  a  tube  ;  the  species  are  marine.  Fistu- 
laria  has  persisted  since  the  eocene.  The  extinct  DERCETIDJE  (Belonor- 
hynchus,  Saurorhamphus  from  the  cretaceous)  show  relations  towards 
Belone. 


ORDER   V.     LOPHOBRANCHII. 

Teleosts  with  hemibranch  ances- 
try ;  body  armored  with  segmented 
bony  armor ;  snout  elongate  ;  tooth- 
less ;  operculum  a  single  plate  ;  gills 
composed  of  small  rounded  tufts ; 
air-bladder  simple,  physoclistous ; 
shoulder  girdle  connected  with  the 
cranium. 

SOLENOSTOMID^E,  large  gill  openings  ; 
two  dorsals  and  ventral  fins.  Solenorhyn- 
chns,  eocene ;  Solenostoma,  eocene  of  Europe 
and  Pacific  Ocean  to-day.  The  females  carry 
the  eggs  and  young  in  a  brood  pouch  formed 
by  the  ventral  fins  and  the  abdominal  or  cau- 
dal surface.  SYGNATHID^E,  appears  in  the 
middle  tertiary;  gill  opening  very  small;  a 
single  dorsal  fin  and  the  ventrals  absent. 
Sygnathus  and  Siphostoma,  pipe-fish,  have 
a  caudal  fin.  In  Hippocampus,  sea-horses, 
the  tail  is  prehensile  and  the  caudal  ab- 
sent. 


FlG.  259.    Sea-horse,  Hippo- 
campus  heptagonuS)  after  Goode. 


ORDER   VI.     ACANTHOPTERYGII    (ACANTHOPTERI). 

Gills  laminate ;  ctenoid  or  cycloid  scales ;  premaxillaries 
forming  the  border  of  the  mouth  ;  shoulder  girdle  connected  to 
the  cranium  by  a  (usually  forked)  post-temporal ;  ventrals  gen- 
erally far  forward,  usually  attached  to  the  shoulder  girdle  by 
means  of  the  pelvis ;  the  anterior  rays  of  pectorals,  ventrals, 
dorsal  or  anal  fins,  usually  osseous  spines.  Here  belong  the 
great  majority  of  marine  teleosts.  Judging  from  vertebral  char- 
acters and  the  suspension  of  the  pectoral  girdle,  they  have  prob- 


TELEOSTS.  259 

ably  developed  from  isospondylous  ancestors,  possibly  by  way  of 
two  or  more  lines,  one  of  them  apparently  being  the  Haplomi. 
Among  the  more  primitive  stocks  are  percesoces  and  the  salmo- 
percae,  and  from  these  and  possibly  other  groups,  the  others  have 
descended.  In  order  that  the  various  families  may  be  arranged 
in  some  accordance  with  their  affinities  these  two  sub-orders 
have  been  somewhat  widely  separated. 

SUB-ORDER  i.     SALMOPERCE. 

Acanthopteri  with  adipose  fin,  dorsals  and  anals  spined ;  ventrals 
abdominal,  ctenoid  scales ;  duct  of  air-bladder  persistent. 

A  structurally  primitive  group  but  no  fossils  known.  Only  genus 
Percopsis,  with  its  centre  in  the  great  lakes.  From  this  sub-order  we  fol- 
low out  first  what  may  be  called  a  percoid  line. 

SUB-ORDER  2.     XENARCHI. 

Acanthopteri  with  a  single  dorsal  fin,  ventrals  thoracic,  air-bladder 
large,  anus  at  the  throat.  Contains  only  the  family  of  pirate  perches  with 
one  species  (Aphredoderus  sayanus]  of  the  U.  S.  Erismatopterus  and 
Amphiplaga  from  the  eocene  of  Wyoming. 

SUB-ORDER  3.     PERCOIDEA. 

Ventrals  with  one  spine  and  five  rays ;  lower  pharyngeals  separate ; 
nostrils  double,  scales  ctenoid.  PERCID^E,  scales  extending  but  a  short 
distance  on  the  vertical  fins ;  lateral  line  continuous ;  palatines  with  teeth  ; 
fresh  water.  Elassoma,  Lepoutis,  (sunfish),  Micropterus  (black  bass), 
Etheostoma  (darters),  Perca  (perch).  Closely  allied  are  the  marine  SER- 
KANNinyE;  Hcemulon,  Priacanthus,  etc.  Perca  dates  from  the  oligocene, 
Serranus  from  the  miocene,  Erismatopterus,  eocene.  SPARID^,  salt-water 
perches,  teeth  of  the  jaws  either  for  cutting,  or  molars,  the  palate  usually 
toothless.  Sargus,  represented  by  the  American  sheepshead,  dates  from 
the  miocene.  Lutjanus.  SCIJENID,E,  Cynoscion,  weak  fish. 

SUB-ORDER  4.     LORICATI   (CATAPHRACTI). 

Body  frequently  armored  with  bony  keeled  scales  or  plates ;  a  bony 
process  —  the  suborbital  stay  —  extends  across  the  cheek  from  the  infraor- 
bital  ring  to  the  preoperculum.  The  SCORP^ENID^E,  or  rock-fishes,  are  the 
most  perch-like ;  Scorp&na,  European ;  Sebastes,  the  Norway  haddock. 
The  living  species  are  marine,  Scorpcena  appearing  in  the  miocene,  Petalop- 
teryx  in  the  cretaceous.  COTTID^E,  sculpins,  body  naked  or  covered  with 
irregularly  arranged  scales;  usually  two  dorsals,  anal  without  spines.  A 
few,  like  Uranidea  and  Cottus,  occur  in  fresh  water;  but  the  larger  forms, 


260  CLASSIFICATION  OF   VERTEBRATES. 

Hemitripterus,  Platycephalus,  etc.,  are  marine.  Lepidocottus,  oligocene. 
The  closely  allied  AGONID^E  are  armored.  DISCOBOLI,  the  skin  naked ; 
bases  of  the  ventral  fins  forming  a  sucker  on  the  lower  surface.  Cyclop- 
terus,  lump-fish  ;  Liparis  ;  TRIGLID^E,  covered  with  scales  or  bony  plates ; 
anal  without  spines ;  two  or  three  rays  of  the  pectoral  separate  from  the 
rest.  Trigla  (appears  in  the  miocene),  the  gurnards,  European ;  Prio- 
notus,  sea-robins,  on  our  coasts.  The  finger-like  free  rays  of  the  pectoral 
are  sensory.  The  DACTYLOPTERID^E  share  with  the  Exoccetidae  (supra} 
the  common  name,  flying-fishes ;  they  have  armored  bodies,  lack  a  lateral 
line,  have  pectorals  enormously  developed,  and  no  palatine  teeth.  Dactyl- 
opterus.  Allied  is  Pegasus,  with  smaller  pectorals  and  elongate  snout, 
from  the  East  Indies. 

SUB-ORDER  5.     XENOPTERYGII. 

No  scales,  no  spinous  dorsal,  gill  arches  reduced,  a  ventral  sucker 
between  the  pectorals  but  not  formed  by  them.  The  only  family  GOBIE- 
SOCID^E  is  somewhat  closely  related  to  the  Batrachidae  and  Cottidae. 
Gobiesox,  American. 

SUB-ORDER  6.     HOLCONOTI. 

Anal  fin  very  long,  scales  cycloid,  lower  pharyngeals  united,  >oung 
brought  forth  alive.  The  surf  perches  (EMBIOTOCID/E)  of  the  Pacific  coast 
form  the  only  members  of  this  group,  which  finds  its  nearest  relatives  in  the 
percoid  fishes  and  in  the  pharyngognaths  (infra}.  Cymatogaster,  Embio- 
toca,  Holconotus,  U.S.;  Ditrema,  Japan. 

SUB-ORDER  7.     PHARYNGOGNATHI. 

Nostrils  double,  lower  pharyngeals  united,  scales  cycloid ;  oviparous. 
A  single  family,  LABRID.E,  of  percoid  affinities,  most  of  the  species  being 
tropical  or  sub-tropical  shore  feeders.  Labrus,  European  wrasses ;  Cteno- 
labrus,  cunners ;  Tautoga ;  Scarus,  parrot-fish.  Allied  are  the  tropical 


FlG.  260.      Gunner,  Ctenolabrns  cicrnlcus,  after  Goode. 


TELEOSTS.  26l 

families,  CHROMID.E,  from  fresh  water,  POMACENTRID^E,  marine,  in  which 
the  nostrils  are  single. 

The  three  sub-orders  following  form  a  blennoid  series  which  have 
sprung  from  a  percoid  stem. 

SUB-ORDER  8.     TRACHINOIDEA. 

Ventrals  thoracic  or  jugular,  nostrils  single,  dorsal  spines  few,  soft 
dorsal  and  anal  long,  body  scaled  or  naked.  This  sub-order  is  best  de- 
veloped in  the  south  temperate  zone.  In  the  deep-sea  CHIASMODONTID/E 
the  body  is  naked,  the  mouth  very  large,  and  two  dorsals  are  present.  The 
species  are  noted  for  their  sharp  teeth  and  enormous  stomach,  swallowing 
fishes  several  times  their  own  size.  The  MALACANTHID^:  are  represented 
off  our  shores  by  the  tile-fish,  Lopholatilus,  of  interesting  history.  TRA- 
CHINID^E  ;  mouth  oblique,  small  conical  teeth,  lateral  line  distinct.  Trach- 
inus,  weevers  ;  Uranoscopus,  Dactyloscopies,  star-gazers  ;  Trachinus  appears 
in  the  eocene. 

SUB-ORDER  9.     BLENNIOIDEA. 

Body  naked  or  with  ctenoid  or  cycloid  scales ;  ventrals  thoracic  or 
jugular,  sometimes  wanting,  the  soft  rays  few  in  number ;  dorsal  fin  long, 
the  spiny  rays  numerous;  anal  long;  tail  homocercal.  BLENNID^,  gill 
openings  normal,  teeth  not  molariform  ;  Blennius*  Gunnellus,  Cryptacan- 
thodes.  ANARRHICHID.E,  posterior  teeth  are  molariform;  Anarrhichas, 
wolf-fishes. 

SUB-ORDER  10.     OPHIDIOIDEA. 

Closely  related  to  the  last,  but  without  spines,  except  sometimes  in  the 
posterior  part  of  the  dorsal ;  tail  diphycercal.  The  eel-pouts  are  all  marine, 
and  occur  in  all  seas.  ZOARCID^E,  ventral  fins  never  filamentous,  some- 
times wanting.  Zoarces.  OPHIDIID^:,  ventrals  slender  filaments,  a  little 
behind  the  eye.  Ophidium.  FIERASFERID.E,  ventrals  lacking;  vent  at 
the  throat.  Several  species  of  Fierasfer  live  as  commensals  with  pearl 
oysters  or  in  the  cloaca  of  holothurians. 

SUB-ORDER  u.     BERYCOIDEA. 

Ventrals  thoracic,  soft  rays  of  pectorals  more  than  5  ;  tail  diphycercal ; 
duct  of  air-bladder,  in  some  cases  at  least,  persistent;  body  naked  or  scaly. 
The  berycoids  are  an  archaic  group,  the  genera  Beryx,  Platycormus,  Holo- 
centrum,  etc.,  appearing  in  the  eocene.  The  nearest  relatives  are  to  be 
sought  in  the  percoids.  The  living  species  are  marine,  and  some  belong 
to  the  deep  seas.  BERYCID^E,  no  barbels  on  the  chin ;  dorsal  is  single. 
HoLOCENTRiDyE,  two  dorsals.  MuLLiD^E,  two  dorsals  and  two  chin 
barbels.  Mullus,  the  surmullets ;  Upeneus,  the  goat-fishes.  Less  certain 
in  its  position  is  the  family  ZEID^E  which  has  some  berycoid  affinities, 
while  it  also  shows  relationships  to  the  squamipinnes. 


262 


CLASSIFICATION  OF   VERTEBRATES. 


SUB-ORDER  12.     SQUAMIPINNES. 

Ventrals  thoracic,  tail  diphycercal  ;  scales  small,  ctenoid  ;  dorsal  fin 
long,  scales  upon  the  soft  portion ;  postorbital  usually  ossified  to  the  skull. 
The  squaniipinnes  have  left  the  main  stem  somewhere  near  the  point  of 
differentiation  of  percoid  and  scombroid  groups.  In  turn  they  have  given 
rise  to  the  plectognaths.  The  order  is  introduced  by  Pomacanthus,  and 
Asineops  in  the  eocene.  In  the  CH^TODONTID/E  the  teeth  are  bristle-like 
and  thick  set.  Chcetodon,  butterfly-fishes  ;  Holacanthus,  emperor-fishes ; 
Toxodon,  the  archer  fish,  has  the  palatines  with  teeth.  All  the  forms  are 
tropical  or  subtropical.  TEUTHID^:,  doctor-fishes,  teeth  incisor-like ; 
caudal  peduncle  is  armed  with  spines  or  plates,  and  frequently  becomes  an 
important  weapon  of  defence.  Teuthis,  tropical  seas. 

SUB-ORDER  13.     PERCESOCES. 

Ventrals  abdominal,  spined ;  dorsal  spines  few,  usually  forming  a  sepa- 
rate fin,  tail  diphycercal,  third  superior  pharyngeal  enlarged,  scales  cycloid. 
The  percesoces  form  another  stem,  arising  probably  from  the  ancestors 
of  the  hemibranchs  and  lophobranchs,  and  close  to  the  synentognaths. 
In  turn  the  scombroids  have  descended  from  some  percesocid  form.  ATH- 
ERINID^E,  or  silver  sides,  lateral  line  lacking,  teeth  small  or  wanting; 
head  and  body  elongate;  species  carnivorous,  mostly  marine;  Atherina 
(appears  in  eocene)  Menidia.  MUGILID^E,  mullets,  herbivorous ;  differ 
from  last  in  the  short  and  broad  head.  Mugil  dates  from  oligocene. 
SPHYR^ENID^E,  lateral  line  distinct;  teeth  strong.  Sphyrcena,  the  barra- 
cudas of  warmer  seas.  Scyllcemus,  cretaceous.  The  OPHIOCEPHALIDVE 
of  the  rivers  of  India  belong  near  the  percesoces,  but  differ  in  absence 
of  spines  from  all  fins.  They  are  capable  of  aerial  respiration,  and  lead 
to  the  labyrinthici.  The  tropical  POLYNEMID^E  show  some  relationships  to 
the  mugilidae.  Polydactylus. 

SUB-ORDER  14.     LABYRINTHICI. 

Dorsal  and  anal  spines  present,  ven- 
trals  thoracic,  lateral  line  interrupted  or 
absent ;  a  complicated  apparatus  of  bony 
laminae  supporting  a  respiratory  mem- 
brane in  the  accessory  branchial  cham- 
ber, by  means  of  which  the  animal  can 
breathe  air.  All  are  tropical.  Anabas 


FlG.     261.       Head 


Anabas, 


showing  the  labyrinthicine  apparatus,    is  said  to  climb  trees.    Osphromenus,  the 
after  Zograff.  gouramy. 

SUB-ORDER  15.     AMMODYTOIDEA. 

Ventrals  absent,  no  spines  in  any  fins,  in  other  respects  much  as  in  the 
percesoces.  A  group  of  uncertain  relations,  placed  here  for  want  of  a 
better  place.  A  single  family,  AMMODYTID^:,  with  cycloid  scales,  no  teeth, 


TELEOSTS.  263 

lateral  line  along  the  side  of  the  back.     Ammodytes,  sand-launces,  common 
on  sandy  shores. 

SUB-ORDER  16.     SCOMBROIDEA. 

Tail  diphycercal,  caudal  usually  strongly  forked ;  ventrals  thoracic ; 
scales  usually  small,  cycloid,  sometimes  absent ;  dorsal  fin  usually  long. 
A  heterogeneous  group,  not  easily  defined ;  developing  in  three  main  lines. 
SCOMBRID^E,  head  normal;  spinous  dorsal  well  developed;  the  dorsal 
divided  up  into  finlets.  Scomber,  mackerels,  first  appear  in  miocene; 
Thynnus,  horse-mackerel,  tunnies  (eocene) ;  Auxis  (miocene),  frigate-mack- 


FlG.  262.     Mackerel,  Scomber  scombrus. 

erel.  TRICHIURID;E,  body  very  long,  tapering  to  a  point;  no  caudal; 
ventrals  rudimentary  or  absent ;  tropical.  Trichinrus,  cutlas-fishes.  The 
allied  Lepidopus  appears  in  the  eocene.  PAL/EORHYNCHID^:,  extinct. 
XIPHIID^:,  bones  of  upper  jaw  prolonged  into  a  sword.  Histiophorus, 
sail-fish,  possesses  scales  and  teeth ;  Xiphias,  sword-fish,  lacks  both.  Xi- 
phiidids  date  from  the  upper  cretaceous. 

CARANGID/E,  pompanos  of  warmer  seas ;  caudal  forked  ;  dorsal  not 
divided  into  finlets  ;  jaws  normal.  Naucrates,  pilot-fish  ;  Seriola,  amber- 
fish  (date  from  the  eocene);  Caranx,  crevalle's  (miocene) ;  Vomer  and  Selene, 
moon-fishes,  with  greatly  compressed  bodies.  Trachinotns,  Platax,  cre- 
taceous. POMATOMID^E,  blue-fish.  CoRYPH^ENiD^E.  dolphins ;  date  from 
eocene.  STROMATEID/E,  with  teeth-like  processes  in  the  oesophagus ; 
Rhouibus,  butter-fish  ;  Palinurichthys,  rudder-fish.  BRAMID^E. 

SUB-ORDER  17.     GOBIOIDEA. 

Dorsal  spines  few  and  weak;  ventrals  thoracic,  usually  close  together; 
soft  dorsal  and  anal  long;  tail  diphycercal.  Over  600  species,  mostly 
marine,  and  of  small  size.  Callionymus  first  appears  in  the  miocene ; 
Gobius  (from  eocene  onwards) ;  Clevelandia.  Typhlogobius  of  Californian 
shores  is  blind. 

SUB-ORDER   18.     DISCOCEPHALI. 

With  the  dorsal  fin  modified  into  a  flat,  transversly  laminated  oval 
sucker  on  the  top  of  the  head;  ventrals  thoracic.  Introduced  by  Opistho- 
myzon  in  the  eocene  of  Switzerland  with  a  smaller  sucker  than  in  recent 


264  CLASSIFICATION  OF   VERTEBRATES. 

forms.     Living   genera,  Echineis  and  Remora,  suck-fishes,  which  attach 
themselves  by  their  suckers  to  fish,  ships,  etc. 


FIG.  263.     Suck-fish  or  remora,  Remora  brachyptera,  after  Goode. 

SUB-ORDER  19.     ANACANTHINI. 

No  spines  in  any  fins,  ventrals  jugular  in  position.  GADID.E,  one  of 
the  most  important  families  of  fishes ;  caudal  fin  present,  scales  cycloid, 
chin  with  barbels  except  in  Merlucius.  Gadus,  cod  and  haddock  ;  Pol- 
lachius,  pollock ;  Microgadus,  tomcod  ;  Lota,  burbot  (fresh  water) ;  Phycis ; 
Merlucius,  hake.  MACRURID^E,  tail  tapering  to  a  point,  without  caudal. 
Macrurus.  The  Anacanthini  are  represented  by  Nemopteryx  in  the  oligo- 
cene,  while  Gadus  and  Phycis  appear  in  the  miocene. 


FIG.  264.     Cod,  Gadus  morrhua,  after  Storer. 

SUB-ORDER  20.     T^ENIOSOMI. 

Body  elongate  and  ribbon  shaped  ;  ventrals  thoracic ;  dorsal  high,  and 
running  the  length  of  the  back ;  mouth  small,  teeth  weak ;  caudal,  when 
present,  directed  upwards.  The  ribbon-fishes  are  deep-sea  forms,  reaching 
a  length  of  15  or  20  feet.  REGALECID^;,  ventrals  reduced  to  a  single 
filament.  Regalecus.  TRACHYPTERID^E,  ventrals  normal  or  wanting. 
Trachypterus  ;  Stylephorus. 

SUB-ORDER  21.     HETEROSOMATA. 

Fins  without  spines  ;  dorsal  and  anals  very  long ;  ventrals  thoracic  ; 
tail  diphycercal ;  head  twisted  so  that  both  eyes  appear  on  the  same  side. 
The  flat  fishes  are  among  the  most  remarkable  of  fishes  from  the  torsion  of 


TELEOSTS. 


the  head.  In  early  life  they  are  sym- 
metrical ;  but  very  soon,  in  some  spe- 
cies before  reaching  the  length  of  an 
inch,  they  turn  over  upon  one  side, 
and  the  eye  of  the  lower  surface  grad- 
ually works  around  to  the  upper  side, 
twisting  the  bones  of  the  skull  in  its 
passage.  The  group  is  nearest  the 
gadoids,  and  probably  these  have  both 
descended  from  some  common  ances- 
tor. Many  of  the  species  are  valuable 
as  food.  All  are  bottom  feeders,  and 
some  come  from  the  deeper  seas. 
PLEURONECTID^:,  preopercular  mar- 
gin distinct ;  mouth  large  or  moderate. 
Hippoglossus,  halibut  of  northern 
seas  ;  Paralichthys  ;  Pseitdopleuro- 
nectes,  winter  flounders ;  Pleuronectes, 
plaice ;  Lophopsetta,  window  pane. 
Psetia  (Rhombus},  turbot  (dates  from 
the  eocene).  SOLEID^E,  preopercular 
covered  by  skin  and  scales  in  front; 
mouth  small  and  twisted.  Achirus, 
American  soles ;  Salea  (dating  from 
the  oligocene),  European  sole.  FIG.  265.  Cranium  of  a  plaice 

(Platessa\  from  Huxley,  showing  the 

distortion  of  the  bones;  the  dotted  line,  ab,  being  the  middle  line.  EpO,  epiotics; 
Er,  frontals;  Etk,  ethmoid;  Pa,  parietal;  Prf,  pref rental;  SO,  supraoccipital^ 
Cr,  position  of  eyes. 


FIG.  266.     Winter 

flounder,  Pseudopleu- 
ronectes  americanits, 
after  Goode. 

SUB-ORDER  22. 


HAPLODOCI. 


Gill  arches  reduced  to   three :    head  large ;    post-temporal   undivided  f 
dorsal  fins  two,   the  dorsals,   pectorals,  and  ventrals  spined  in  front,   the 


266  CLASSIFICATION  OF   VERTEBRATES. 

ventrals  jugular  ;  scales  cycloid  or  wanting.  A  single  family,  BATRACHID^E, 
mostly  from  warmer  seas.  Batrachus,  toad-fish  ;  Thalassophryne,  poison 
toad-fish.  Porichthys,  midshipman,  of  Pacific  coast,  with  numerous  dermal 
organs,  in  structure  resembling  phosphorescent  organs,  but  not  luminous. 

ORDER   VII.     PEDICULATI. 

Pectorals  broad,  suspended  by  an  <  arm '  formed  by  the 
elongation  of  the  basilar  bones  ;  head  and  anterior  part  of  body 
very  large,  without  scales ;  spinous  dorsal  far  forward,  the 
spines  often  like  tentacles  ;  gill  opening  a  small  foramen  in  or 
near  the  axilla ;  ventrals  jugular.  The  most  specialized  of 
fishes,  with  possibly  an  haplodocan  ancestry. 

LOPHIID^E,  large  mouth ;  strong  teeth ;  ventrals  present.  Lophius, 
angler  or  goose-fish  ;  the  genus  dates  from  the  eocene.  ANTENNARIID.E, 
pectorals  bent  at  an  elbow-like  angle  ;  ventrals  jugular ;  Pterophryne,  in 
gulf-weed.  Antennarius.  MALTHID^E,  mouth  small,  usually  inferior. 
Malthe,  sea-bats. 

ORDER   VIII.     PLECTOGNATHI. 

Bones  of  upper  and  lower  jaws  each  co-ossified  ;  post- 
temporal  simple  ;  ventrals  reduced  or  wanting  ;  gills  pectinate ; 
gill  opening  narrow,  just  in  front  of  pectorals  ;  spinous  dorsal 
small  or  wanting.  The  plectognaths  have  arisen  from  near  the 
squamipinnes  (above),  the  teuthids  being  very  near  the  trigger 
fishes. 


FIG.  267.     Swell-fish,  Chilomycterus  geometricus,  after  Goode. 

SCLERODERMI,  jaws  with  distinct  teeth  ;  spinous  dorsal  present ;  body 
with  scales  or  movable  plates.  Batistes,  file-fish  or  trigger-fish ;  Mona- 
canthus,  file-fishes;  Acanthoderma,  eocene;  Altitera,  unicorn-fish.  Os- 
TRACODERMI,  jaws  with  distinct  teeth ;  body  enclosed  in  a  three,  four,  or 


DIPNOI. 


267 


five  angled  box  composed  of  polygonal  body  plates,  firmly  united.  Ostracion 
Lactophrys,  trunk-fishes.  Ostracion  first  appear  in  the  eocene.  GYMNO- 
DONTI,  spinous  dorsal  lacking ;  scales  spiniform  or  absent ;  jaws  with 
enamelled  plates,  but  without  distinct  teeth.  Tetrodon,  Diodon,  Chilomyc- 
terus,  etc.,  swell-fishes  or  globe-fishes,  etc.,  the  common  names  arising 
from  the  powers  of  inflation  possessed  by  them.  Mola  (Orthagoriscus), 
sun-fish,  the  most  bizarre  of  fishes,  seemingly  but  a  large  head  with  fins 
attached.  Fossils  doubtfully  referred  to  Mola  occur  in  the  British  creta- 
ceous ;  Diodon  appears  in  the  eocene. 


FlG.  268.     Sun-fish,  Mola  rotunda,  after  Putnam. 


SUB-CLASS    III.     DIPNOI    (DIPNEUSTES). 

Fishes  with  partially  ossified  cartilage,  numerous  membrane 
bones,  and  persistent  notochord  ;  skull  autostylic  ;  a  membra- 
nous operculum  ;  tail  diphycercal  ;  paired  fins  archiptergial  or 
reduced  ;  heart  with  multivalvular  conus,  a  spiral  valve  in  in- 


268 


CLASSIFICATION  OF   VERTEBRATES. 


testine ;  a  cloaca  present,  air-bladder  single  or  paired,  function- 
ing as  a  lung. 

The  dipnoi,  or  lung-fishes,  are  frequently  regarded  as  belong- 
ing to  the  ganoids,  and  have  attracted  great  attention  from  the 
fact  that  they  are  often  considered  as  intermediate  in  position 
between  the  lower  fishes  and  the  amphibia.  In  past  time  the 
group  was  richly  represented ;  but  in  the  existing  fauna  of  the 
earth  but  four  species  are  known,  these  having  that  wide  and 
discontinuous  distribution  so  frequently  characteristic  of  the 
survivors  of  an  ancient  group. 

They  all  have  an  elongate  fish-like  or  eel-like  body,  covered 
in  the  recent  species  with  overlapping  cycloid  scales  ;  while  in 
the  fossils  ganoid  scales  frequently  occurred.  The  fins  are  sup- 
ported by  horny  dermal  rays.  The  axial  skeleton  consists  of 
the  persistent  notochord,  around  which  (except  in  the  tail  region 
of  some  forms)  vertebral  centra  are  not  developed,  segmenta- 
tion being  shown  only  in  the  neural  arches  and  ribs. 


FIG.  269.  Skull  of  ProtopteruS)  after  Wiedersheim.  #,  angulare ;  </,  dentary ; 
e,  otic  capsule;  /,  skeleton  of  fore  limb  ;  fp,  frontoparietal  ;  g,  external  gills;  //, 
hyoid ;  hr,  head  rib ;  n,  nasal ;  nc,  nasal  capsule ;  //>,  palatopterygoid  ;  q,  quadrate ; 
s,  supracranial  bone;  sq,  squamosal ;  so,  supraoccipital ;  /- F7,  branchial  arches. 


The  skull  consists  of  the  largely  persistent  chondrocranium 
plus  a  number  of  membrane  bones  not  easily  homologized  with 
those  of  other  vertebrates.  In  the  chondrocranium  exoccipitals 
alone  are  developed.  In  the  lower  groups  the  membrane  bones 
are  very  numerous  ;  but  in  existing  forms,  as  in  the  extinct 


DIPNOI.  269 

arthrodira,  they  are  few.  In  the  existing  species  parasphenoid, 
vomers,  palatoquadrate  and  squamosal,  as  well  as  dentary,  angu- 
lar and  opercular  in  the  lower  jaw,  are  more  or  less  certainly 
to  be  recognized  ;  but  beside  these  there  are  several  bones  in  the 
cranial  roof  which  are  not  to  be  homologized  with  those  in  other 
groups. 

The  operculum  is  supported  by  bones  (operculum,  inter- 
operculum),  and  the  hyoid  and  (four  or  five)  branchial  arches  are 
cartilaginous.  The  pectoral  and  pelvic  arches  are  cartilaginous, 
the  former  with  membrane  bones  of  the  fish-type  (p.  174)  weakly 
developed.  The  pelvis  consists  of  a  median  plate  with  (in  recent 
forms)  three  anterior  horns.  The  fins  themselves  are  very  prim- 
itive, and  consist  of  an  axial  portion,  from  which,  in  Ceratodus, 
biserial  cartilaginous  fin  rays  arise.  In  many  fossils  the  ante- 


FlG.  270.     Restoration  of  Dinichthys,  from  Dean. 

rior  part  of  the  body  is  enclosed  in  a  strong  armor  of  bony  der- 
mal plates,  there  being  a  hinge  between  the  dorsal  plates  and 
the  base  of  the  skull. 

The  brain  differs  from  that  of  the  teleostomes  in  the  ner- 
vous character  of  the  cerebral  mantle.  The  two  hemispheres  are 
united  in  Ceratodus,  but  in  Protopterus  they  are  distinct  back  to 
the  anterior  commissure.  The  mid  brain  is  paired  in  Ccmtodus, 
simple  in  Protopterus ;  the  cerebellum  is  but  a  small  transverse 
fold.  The  pineal  structures  have  a  long  stalk,  while  the  envel- 
opes of  the  brain  are  richly  developed,  and  in  Protopterus  these 
enter  above  the  fossa  rhomboidalis  into  close  connection  with 
the  endolymphatic  system  of  the  ear.  An  optic  chiasma  is 
present. 

The  teeth  are  few  in  number,  and  are  apparently  formed  by 
the  fusion  of  several  primitive  teeth.  Of  these  there  is  a  pair 
of  larger  grinding  plates  borne  on  the  palatopterygoids,  a  much 


2/0 


CLASSIFICATION  OF   VERTEBRATES, 


FIG.  271.  Tooth 
of  ceratodan,  Sageno- 
dus,  after  Woodward. 


smaller  pair  on  the  vomerine  region,  while  the  lower  jaw  has 
a  pair  on  the  splenial  region.  The  alimentary  canal  is  nearly 
straight,  and  is  char- 
acterized by  the  pres- 
ence of  a  well-devel- 
oped spiral  valve 
(Fig.  40)  in  the  in- 
testine. Behind,  the 
intestine  empties  into 
a  cloaca,  which  also  receives,  besides  the 
urogenital  ducts,  median  or  paired  pori 
abdominales.  There  are  three  (JProtop- 
terus)  or  four  (Ceratodus)  l  pairs  of  in- 
ternal gills,  and  besides,  in  the 
former,  external  gills  (Fig.  269). 
Besides  these,  there  are  present  in 
each  swim-bladders  which  also  have 
respiratory  functions.  In  Cera- 
todus this  lung  is  single,  in  Pro- 
toptcrus  it  is  paired  ;  but  in 
both  its  duct  or  ducts  arise 
from  the  ventral  surface  of  the 
pharynx.  Internally  these  or- 
gans are  sacculated,  while  the 
blood  comes  to  it  by  true  pul- 
monary arteries,  which  arise 

either  (Ceratodus)  from  the  pOS-         FIG.   272.      Heart   and   anterior  part 

terior  branchial,  or  (Protopterus)    of  the  lunss  of  Ceratodus,  after  Rose. 

,.  a,   aortic   arches   and  auricle;    <:,  post- 

from  the  radices  aortae.  cardinal  vein  .  ^  conus;  ^  h     tic 

The  heart  has  both  the  sinUS     veins:   ;,  lung;  /,  jugular  vein;  ji,  in- 

and  the  atrium  partially  divided    ferior  jugular  vein;  oe,  oesophagus;  /, 

.'  .  .  i   i    r^   i     i  i  pulmonary  arteries;   s,   subclavian  vein 

into  right  and  left  halves  by  an    and  sinu/venosus< 
incomplete    septum,    thus    fore- 
shadowing the  conditions  found  in  the  amphibia,  while  a  true 
atrio-ventricular  valve  is  lacking.     The  con  us  is  spirally  twisted, 
and  contains  several  rows  of  valves,  and  in  Ceratodus  is  partially, 
in  Protopterus  completely,  divided  into  venous  and  arterial  halves. 

1  Ceratodus  also  has  a  hyoid  pseudobranch  (p.  23) . 


DIPNOI.  2/1 

In  the  venous  system  the  most  marked  advance  is  the  presence 
of  a  postcava,  while  but  a  single  (left)  postcardinal  comes  to 
complete  development.  A  renal  portal  system  is  present. 

The  mesonephros  is  elongate  in  Protopterus,  short  in  Cerato- 
dus ;  and  nephrostomes  are  lacking  in  the  adult.  Its  duct  is 
thick  walled,  and  is  apparently  a  Wolffian  duct,  although  em- 
bryological  evidence  is  as  yet  lacking.  The  gonads  are  elon- 
gate, and  attached  to  the  lateral  parts  of  the  mesonephros.  The 
oviducts  are  elongate  and  contorted,  and  open  into  the  coelom 
far  forward  by  narrow  ostia.  Posteriorly  they  unite  just  in 
front  of  the  cloaca.  In  Ceratodus  no  vasa  deferentia  occur,  the 
spermatozoa  apparently  passing  out  through  the  pori  abdomi- 
nales.  In  Protopterus  a  well-developed  duct  occurs  in  connec- 
tion with  either  testis,  each  passing  behind  into  the  rudimentary 
Miillerian  duct,  and  thence  by  a  common  trunk  into  the  cloaca. 
The  cloaca  also  bears  an  azygos  diverticulum  (Fig.  40),  usually 
regarded  as  an  urinary  bladder  (cf.  however,  the  rectal  gland  of 
elasmobranchs). 

Of  the  development  of  Protopterus  nothing  is  known.  The 
segmentation  and  external  development  of  Ceratodus  have  been 
studied,  and  show  striking  similarities  to  that  of  Petromyzon, 
and  especially  to  the  amphibia.  The  egg  undergoes  a  total  but 
unequal  segmentation,  while  gastrulation  is  effected  by  over- 
growth as  in  the  amphibia,  the  result  being,  as  in  that  group  — 
the  formation  of  an  elongate  primitive  groove,  on  either  side  of 
which  the  medullary  folds  arise.  These  close  in,  and  gradually 
the  embryo  arises  as  a  ridge  above  the  yolk.  So  far  as  is  known 
no  metamorphosis  occurs. 

ORDER    I.     ARTHRODIRA. 

Body  in  front  covered  with  large  bony  plates,  a  dorsal  pair 
articulating  by  a  hinge  joint  with  the  cranium  ;  paired  fins  rudi- 
mentary or  absent ;  pelvis  represented  by  a  pair  of  club-shaped 
plates. 

The  relationship  of  the  arthrodira  to  the  other  dipnoi  is  not 
beyond  question.  The  group  is  restricted  to  the  palaeozoic 
rocks,  and  remains  are  abundant  in  Europe  and  in  America. 


272 


CLASSIFICATION  OF   VERTEBRATES. 


Coccostens  appears  in  the  Silurian  of  Europe,  and  occurs  in  the 
Devonian  of  Ohio.     In  the  Devonian  and  carboniferous  of  Ohio 


FIG.  273.  Coccosteus,  restored,  after  Woodward,  from  Dean.  A,  articulation 
of  head  with  trunk  ;  DB,  basalia ;  DR,  radialia  of  dorsal  fin  ;  //,  haemal  arch  and 
spine;  MC,  lateral  line  canals;  JV,  neural  arch  and  notochord;  (7,  unpaired  plate; 
VB,  VR,  basalia  and  radialia  of  ventral  fin. 

also  occur  some  gigantic  forms  belonging  to  the  genera  Dinich- 
thys,  Titanichthys,  and  Macropetalichthys,  the  latter  genus  occur- 
ring in  Germany  as  well. 


FIG.  274.  Dorsal  wall  of  skull  of  Dinichthys,  after  Claypole.  £,  ethmoid  ; 
£0,  exoccipital;  F>  frontal;  M,  marginal;  N,  nasal;  P,  parietal;  PO,  postorbital ; 
PR,  preorbital ;  S,  supraoccipital ;  sensory  canals  dotted.  (The  homologies  of  some 
of  the  bones  with  the  similarly  named  elements  in  other  groups  is  doubtful.) 

ORDER   II.     SIRENOIDEA. 

Body  never  with  bony  plates,  usually  covered  with  cycloid 
scales ;  paired  fins  archipterygial ;  pelvis  a  single  median  plate. 
The  recent  forms  are  subdivided  into  MONOPNEUMONIA  and 


DIPNOL  273 

DIPNEUMONIA.  In  the  first,  typified  by  the  Australian  genus 
Ccmtodus,  there  is  but  a  single  air-bladder  (lung),  and  the  fins 
have  the  secondary  rays  of  the  archipterygium  (Fig.  185)  well 
developed.  Ceratodus  forsteri  of  Australia  attains  a  length  of 
five  feet ;  it  lives  in  fresh  water  in  places  where  it  is  apt  to  be- 
come stagnant,  and  at  such  times  calls  its  lung  into  function. 


FIG.  275.     Lung-fish,  Protopterus  annectans,  from  Boas. 

The  genus  Ceratodus  also  occurs  fossil  in  the  triassic  and  Juras- 
sic of  Europe,  India,  and  Colorado,  the  peculiar  dental  plates 
being  very  characteristic  (Fig.  271).  The  Dipneumonia  have 
two  air-bladders,  and  the  paired  fins  retain  only  the  axial  part  of 
the  archipterygial  skeleton  (Fig.  269).  The  living  genera  are 
Protopterus  from  African  rivers,  and  Lepidosiren  from  South 
America.  At  the  time  of  drought  the  African  form  burrows 
into  the  mud  at  the  bottom  of  the  pools 
where  it  lives,  and  by  the  aid  of  the  mucus 
from  its  body  forms  the  earth  into  a  '  co- 
coon,' in  which  it  lives  in  a  state  of  sus- 
pended animation  until  the  return  of  the 
rainy  season. 

Allied  to  these  living  animals  are  a  num- 
ber of  fossil  forms  characterized  by  the  pres- 
ence of  numerous  plates  in  the  cranial  wall. 
These  occur  in  the  palaeozoic  rocks.  In 
Dipterus  and  Phaneropleuron  from  the  De-  FlG-  27°".  Dorsal 

r      T-.  i       A  •  •          i          view    of   skull    of    Dip' 

voman    of    Europe    and    America,   jugular   ,,rw>  after  Pander. 

plates  are  present ;  in  Ctenodus  and  Sageno- 

dus,  from  the  carboniferous  of  both  hemispheres,  jugulars  are 

lacking. 


274  CLASSIFICATION  OF   VERTEBRATES. 

CLASS  II.     AMPHIBIA  (BATRACHIA). 

o 

Ichthyopsida  in  which  lungs  are  present  and  the  gills  are 
usually  lost  in  the  adult.  Median  fins  never  supported  by 
dermal  rays ;  paired  appendages  in  the  shape  of  legs ;  body 
without  scales  except  in  caecilians  and  stegocephals ;  a  stapes 
always  present,  and  an  Eustachian  tube  in  the  higher  forms  ; 
nostrils  communicating  posteriorly  with  the  mouth ;  a  post- 
cava  always  present. 

The  amphibia  are  readily  distinguished  from  the  fishes  by 
the  absence  of  paired  fins,  their  place  being  taken  in  most 
forms  by  legs  built  upon  the  pentadactyl  type,  like  those  of 
amniotes.  Occasionally,  as  in  Siren,  one  pair  of  limbs  may 
be  absent,  or  again,  as  in  the  gymnophiona  and,  among  the 
stegocephals,  the  aistopoda,  both  pairs  are  lacking.  Median 
fins,  confined  to  the  caudal  region,  occur  in  the  young  of  all,  and 
in  the  adults  of  many  aquatic  species,  but  these  are  never  sup- 
ported by  dermal  rays,  while  the  tail  is  diphycercal  in  character. 

The  skin  is  largely  without  cuticular  structures,  but  its 
outer  layers  become  cornified  and  are  periodically  shed.  The 
deeper  layers  contain  numerous  glands ;  the  secretions  of  some 
of  these  are  acrid,  and  in  some  cases  poisonous  ;  upon  these 
depends  the  safety  of  these  otherwise  unprotected  animals.  In 
some  cases  the  skin  is  smooth,  in  others  it  is  roughened  and 
covered  with  warts,  in  part  due  to  local  thickenings,  in  part  to 
the  presence  of  these  defensive  glands.  Epidermic  nails  occur 
on  the  toes  of  a  few  forms. 

In  a  few  living  species  of  anura,  calcareous  deposits  occur 
in  the  dermis,  and  occasionally  (Ceratophrys,  etc.)  bony  dorsal 
plates  may  be  developed  in  the  same  layer.  In  most  gymno- 
phiona semicircular  dermal  scales  envelop  the  body,  giving  it 
a  ringed  appearance  externally.  This  dermal  skeleton  was 
better  developed  in  the  extinct  stegocephals,  where  we  usually 
find  from  one  to  three  large  ventral  bony  plates  and  a  number 
of  smaller  ventral  scales,  but  occasionally  this  armor  extended 
over  the  back  and  limbs. 

The  mouth  is  always  terminal  ;  and  teeth,  when  present, 
occur  on  the  margins  of  the  jaws  (premaxillaries  and  maxilla- 


AMPHIBIA.  27$ 

ries),  and  usually  upon  the  vomers.  In  the  urodeles  teeth  may 
also  occur  upon  the  palatines,  and  occasionally  upon  the  para- 
sphenoid.  In  all  cases  they  are  firmly  anchylosed  to  the  sup- 
porting bones.  In  the  anuran  tadpoles  the  jaws  are  covered 
with  horny  plates.  In  the  urodeles  the  tongue  is  rudimentary. 
It  is  lacking  in  one  division  (aglossa)  of  the  anura ;  but  in  the 
rest  it  is  fixed  in  front,  the  bifid  free  end  being  turned  back 
in  the  mouth.  It  is  capable  of  extension  beyond  the  jaws,  and, 
covered  with  adhesive  mucus,  is  used  in  the  capture  of  food. 

The  central  nervous  system  has  all  of  its  parts  lying  in  one 
plane.  The  cerebrum  is  larger,  and  differs  from  that  of  all 
fishes,  even  of  the  dipnoi,  in  the  greater  development  of  the 
pallium.  The  olfactory  lobes  are  in  close  connection  with  the 
cerebral  hemispheres.  The  cerebellum,  on  the  other  hand, 
is  reduced  to  a  small  transverse  fold.  The  Gasserian  and  acus- 
tico-facialis  ganglia  are  distinct  in  urodeles  and  gymnophiona ; 
but  in  the  anura  they  are  closely  united,  and  the  roots  of  the 
corresponding  nerves  are  not  distinguishable  by  ordinary 
dissection.  There  is  a  similar  union  of  the  glossopharyngeal 
and  vagus  ganglia,  and  the  common  trunk  of  the  ninth  and 
tenth  nerves  passes  from  the  skull  by  a  single  foramen.  In  all 
aquatic  forms  and  larvae  the  lateralis  branches  of  the  seventh 
and  tenth  nerves  is  well  developed;  but  with  the  assumption 
of  a  terrestrial  life  these  are  lost,  together  with  the  lateral  line 
system  which  they  supply  (p.  67). 

The  nasal  passages  form  complete  tubes,  opening  into  the 
oral  or  pharyngeal  cavities  by  internal  nares  or  choana  in  its 
roof.  Connected  with  the  olfactory  organs  are  well-developed 
organs  of  Jacobson  (p.  77).  The  epiphysial  structures  do  not 
extend  beyond  the  skull  in  urodeles  or  gymnophiona  ;  but  in  the 
anura  the  parietal  eye  lies  between  the  skull  and  the  skin,  all 
connection  between  it  and  the  brain  being  lost.  In  the  stego- 
cephals  there  is  a  large  parietal  foramen  in  the  skull,  which 
is  interpreted  as  having  contained  a  functional  parietal  eye. 
The  ears  show  an  advance  upon  those  of  the  fishes  in  the 
development  of  a  distinct  lagena,1  while  the  spiracular  cleft, 

1  The  lagena  is  the  seat  of  audition,  and  recent  experiments  show  that  hearing  first 
appears  in  the  amphibia. 


2/6 


CLASSIFICATION  OF   VERTEBRATES. 


in  the  anura,  enters  into  the  accessory  auditory  structures, 
forming  an  Eustachian  tube  leading  from  the  tympanic  cavity 
to  the  pharynx.  A  stapes  is  also  developed,  and  in  the  anura 
this  is  joined  to  a  columella  (possibly  derived  from  the  hyoman- 
dibular)  which  stretches  across  the  middle  ear  to  the  tym- 
panic membrane.  In  the  urodeles  and  caecilians  the  columella 
and  Eustachian  tube  are  absent,  and  frequently  the  stapes 
articulates  directly  with  the  quadrate. 

The  oral  and  pharyngeal  cavities  are  ciliated  ;  and  into  them 
open,  in  front,  the  internal  nares,  and  behind,  the  slit-like  glottis, 
communicating  with  the  more  or  less  elongate  trachea.  In  the 
young,  and  in  phanerobranchs  and  derotremes,  gill  slits  occur 
in  the  pharyngeal  region.  Of  these,  three  open  to  the  exterior, 
while  one  (or  in  some  cases  two)  pouches  behind  these  never 
break  through. 

The  alimentary  tract  may  be  nearly  straight  in  the  elongate 
forms,  or  be  greatly  convoluted  in  those  with  shorter  bodies, 
the  convolutions  reaching  their  extreme  in  the  herbivorous  tad- 
poles of  the  anura.  The  rectum  is  short,  and  opens  into  the 
cloaca.  The  liver  is  two-lobed,  and  in  the  anura  the  left  lobe  is 
more  or  less  completely  sub-divided.  The  pancreas  is  flattened 
and  lobulated. 

In  the  young  of  all  external  gills  occur,  and  these  may  per- 
sist throughout  life  (perennibranchs).  These  gills  are  ecto- 

dermal  in  origin,  and  arise  as 
outgrowths  from  the  side  of  the 
neck  before  the  gill  slits  break 
through.  Usually  they  are  more 
or  less  branched  and  feathered, 
but  in  Concilia  compressicauda 
they  are  large  sacs.  The  ento- 
dermal  gills  are  a  later  appear- 
ance, and  arise  from  the  walls  of 
the  gill  clefts.  These  clefts  at 
first  open  freely  to  the  exterior ; 
but  in  the  adults  of  most  they 

become  closed,  remaining  permanently  open  only  in  the  peren- 
nibranchs and  derotremes.  In  the  anuran  tadpole  an  oper- 


FIG.  277.  Head  of  young  Die- 
myctylus  viridescens,  showing  lateral 
line  openings  and  remains  of  gill  clefts. 


AMPHIBIA.  277 

cular  fold,  traces  of  which  are  found  in  some  urodeles,  grows 
back  over  the  gill  slits  in  such  a  way  as  to  enclose  them  in  an 
extrabranchial  or  atrial  chamber  on  either  side,  the  two  com- 
municating by  a  passage  beneath  the  throat,  and  opening  to  the 
exterior  usually  by  a  single  opening  upon  the  left  side.1 

The  lungs,  which  are  absent  from  several  salamandars  which 
respire  by  means  of  the  skin,  are  thin-walled  sacs,  which  may  be 
either  smooth  internally  or  folded  into  alveoli  and  infundibula 
(Fig.  33).  The  shape  is  correlated  with  that  of  the  body, 
elongate  in  the  longer  species,  shorter  in  the  more  compact 
forms.  Occasionally  (gymnophiona,  Amphiuma)  the  left  lung  is 
small  or  rudimentary.  The  trachea  may  be  long  or  the  bronchi 
may  unite  just  behind  the  glottis.  The  glottis  is  supported  by 
a  pair  of  arytenoid  cartilages,  and  in  the  anura  a  ring-like  cricoid 
is  added. 

In  many  stegocephals  the  vertebral  column  is  poorly  de- 
veloped, the  centra  being  sometimes  represented  by  pleuracen- 
tra  and  hypocentra  arcale  and  pleuralia  (rhachitomous  type, 
p.  i  36)  ;  or  again  by  an  embolomerous  condition  where  centralia 
and  intercentralia  alternate.  These  two  conditions,  sometimes 
used  as  a  basis  of  sub-division,  may  occur  in  the  same  species. 
In  the  living  species  the  centra  are  well  developed,  and  are 
either  amphiccelous  (perennibranchs,  gymnophiona,  and  some 
salamanders),  opisthoccelous  (most  salamanders  and  aglossate 
anura),  or  precocious  (most  anura).  The  number  varies  from 
9,  plus  the  urostyle,  in  living  anura  to  250  or  more  in  the  gym- 
nophiona. At  most  but  four  regions  can  be  recognized,  —  cer- 
vical, trunk,  sacral,  and  caudal  ;  the  single  cervical  is  without 
ribs,  but  bears  in  front  an  odontoid  process  derived  from  an  an- 
terior vertebra  which  early  fuses  with  the  skull.  There  is  also 
a  single  sacral  vertebra  in  all  except  one  group  of  fossil  anura 
(Palseobatrachidae),  where  there  are  two.  In  the  urodeles  the 
vertebrae  bear  dia-  and  parapophyses  (p.  141),  but  in  the  anura 
only  the  diapophysis  persists. 

The  ribs  are  small,  bicipital  in  urodeles  and  gymnophiona, 
anchylosed  to  the  vertebras  in  the  anura.  In  the  stegocephals 
they  are  larger,  but  in  no  case  do  they  reach  the  ventral  surface. 

1  Paired  openings  occur  in  the  agl^g^^^^ij^^hal  opening  in  a  few  forms. 
jf^'     OF  THf  ^ 

{    UNIVERSITY    1 


CLASSIFICATION  OF   VERTEBRATES. 


FIG.  278.     Skeleton 
of  Necturus. 


The  sternum  is  lacking  in  the  gymno- 
phiona ;  in  the  urodeles  and  arciferous 
anura  it  is  a  median  plate  grooved  to 
receive  the  epicoracoids  in  front.  In 
the  firmisternous  anura  the  sternum  ex- 
pands in  front  of  the  procoracoids  and 
clavicles  into  an  omosternum,  behind  the 
coracoids  to  a  xiphisternum  which  may 
be  partially  ossified. 

The  skull  is  noticeable  for  the  great 
extent  to  which  .the  chondrocranium 
persists,  and  for  the  wide  interval  be- 
tween the  trabeculae.  This  persistence 
of  cartilage  accounts  for  the  small  num- 
ber of  cartilage  bones  found  in  all 
groups  except  the  gymnophiona.  Thus 
in  the  anura  only  a  prootic  occurs  in  the 
auditory  region  ;  in  the  urodeles  an  opis- 
thotic  is  added.  In  the  occipital  region 
there  usually  occur  but  the  two  exoccip- 
itals,  each  bearing  an  occipital  condyle. 
The  quadrate  forms  the  sole  suspensor 
of  the  jaw,  and  is  more  or  less  closely 
connected  with  the  otic  capsule.  If  a 
hyomandibular  be  present,  it  is  modified 
into  the  stapes.  In  the  urodeles  no  eth- 
moid ossification  occurs,  while  an  orbito- 
sphenoid  is  the  only  bony  element  in 
the  trabecular  region.  In  the  anura  a 
ring-like  sphenethmoid  occurs  (os  en 
ceinture).  In  the  gymnophiona  the  eth- 
moid is  very  large  and  has  large  lateral 
wings. 

The  Mpmbrane  bones  are  more  nu- 
merous ii&ie  gymnophiona,  stegoceph- 
als,  and  aWra  than  in  the  urodeles,  the 
skull  being  very  complete  in  the  first 
two  groups,  while  in  the  anura  a  large 


AMPHIBIA. 


2/9 


gap  appears  between  the  cranium  and  the  quadrat oj  ugal-malar 
arch.  This  latter  arch  is  entirely  absent  in  the  urodeles.  The 
roof  of  the  mouth  is  formed  by  vomers,  palatines,  and  a  para- 
sphenoid,  the  latter  element  not  reappearing  in  the  higher 
groups.  In  the  caecilians  the  parasphenoid  fuses  indistinguish- 
ably  with  occipital  elements.  All  of  these  bones  may  bear  teeth, 
as  may  also  premaxillaries  and  maxillaries,  the  latter  element 


FIG.  279.  Skull  of  Ichthyophis  glutinosus,  after  the  Sarasins.  b,  basal,  com- 
posed of  the  coalesced  parasphenoid  and  the  occipitals ;  £,  ethmoid  ;  /,  frontal ; 
/,  jugal  ;  mpy  maxillopalatine  ;  n,  nasal  ;  /,  parietal;  pf,  prefrontal ;  pm,  premaxil- 
lary ;  po,  postf rental ;  pt,  pterygoid  ;  s,  suspensorium ;  s/,  stapes;  /  (in  front), 
turbinal ;  (behind)  tentacular  groove. 

occasionally    being    absent.      The    quadrate    is    overlaid    by   a 
squamosal. 

In  the  shoulder  girdle  coracoid,  procoracoid,  and  scapular 
elements  are  formed ;  in  the  urodeles  the  procoracoid  usually 
extends  directly  forward,  but  in  the  anura  the  ventral  ends  are 
connected  by  an  epicoracoid,  and  the  procoracoid  is  more  or  less 
completely  replaced  by  a  membrane  bone,  —  the  clavicle.  The 
amount  of  ossification  varies  indifferent  forms.  The  pelvis  is 
characterized  by  the  developmem  of  the  ilium,  which  is  very 
strong  in  the  anura.  Ventral^  there  is  frequently  a  continu- 
ous ischiopubic  plate  in  which  a  distinct  pubis  rarely  ossifies. 
Epipubic  processes  are  common  in  the  urodeles.  The  limbs 


280  CLASSIFICATION  OF   VERTEBRATES. 

are  typically  pentadactyl,  with  primitively  a  simple  carpus  and 
tarsus.  In  the  anura  there  is  a  fusion  of  ulna  and  radius,  while 
in  the  hind  foot  the  proximal  elements  of  the  tarsus  (astragalus 
and  calcaneum)  become  greatly  elongate. 

In  the  heart,  which,  except  in  the  gymnophiona,  is  far  ante- 
rior, there  is  always  a  single  ventricle.  In  the  perennibranchs 
and  lungless  salamanders  the  auricles  are  incompletely  sepa- 
rated, but  in  the  other  amphibia  two  distinct  auricles  occur. 
The  right  auricle  receives  venous  blood,  while,  when  the  lungs 
are  functional,  the  left  receives  arterial  blood.  In  the  lungless 
forms  the  pulmonary  vein  is  absent.  In  the  gymnophiona  two 
rows  of  valves  occur  in  the  conus,  but  elsewhere  this  region  is 
reduced  to  a  single  circle  of  semilunar  valves.  The  bulbus  is 
well  developed,  and  in  the  anura  contains  a  longitudinal  valve 
which,  by  changes  in  position,  directs  the  first  blood  to  leave  the 
ventricle  (arterial  blood)  into  the  carotids  and  the  general  cir- 
culation, while  the  venous  blood  which  follows  it  is  sent  into  the 
pulmonary  artery,  and  thence  to  the  lungs. 

Four  pairs  of  aortic  arches  appear  in  the  later  larvae,  the 
blood  at  first  passing  through  them  to  the  gills,  and  thence  to 
the  dorsal  aorta.  With  the  metamorphosis  the  branchial  circu- 


FIG.  280.  Diagram  of  venous  circulation  in  an  amphibian,  av,  anterior 
abdominal  vein;  alt  caudal  vein;  cv,  posterior  cardinal  veins;  //,  hepatic  veins;  ht, 
heart;  z,  interrenal  vein;  iv,  iliac  vein;  /,  jugular  vein;  /',  mesonephros;  />,  portal 
vein ;  pc,  postcava ;  s,  subclavian  vein. 

lation  is  lost ;  but  in  the  urodeles  all  four  arches  persist,  the 
first  supplying  the  carotids,  the  second  and  third  forming  the 
radices  aortae,  while  the  fourth  go  to  the  lungs.  In  the  gymno- 
phiona and  anura  the  third  of  these  disappears.  In  the  venous 
system  the  most  marked  feature  is  the  appearance  of  a  hepatic- 
portal  system  (p.  192)  lacking  in  the  other  ichthyopsida. 

The  pronephros  is  a  transitory  organ.      It  is  confined  to  two 


AMPHIBIA.  28l 

(most  urodeles)  or  three  somites  (anura)  or  several  segments 
(gymnophiona,  Amphiuma.)  It  is  replaced  later  by  the  perma- 
nent mesonephric  kidney,  the  anterior  end  of  which  in  the  male 
becomes  subsidiary  to  reproductive  purposes  (p.  129).  In  the 
gymnophiona  it  is  markedly  segmental.  The  ovaries  are  long 
bands  in  the  gymnophiona,  elongate  sacs  in  the  urodeles,  and 
shorter  sacs  divided  by  transverse  partitions  in  the  anura.  The 
eggs,  in  their  passage  through  the  Miillerian  ducts,  become  en- 
veloped in  a  gelatine  which  swells  in  contact  with  the  water. 
The  Miillerian  duct  always  persists  in  the  male.  The  sperm 
passes  through  the  anterior  part  of  the  kidney,  and  thence  to 
the  exterior  by  the  way  of  the  urinary  duct.  In  many  urodeles 
it  becomes  enclosed  in  packets  (spermatophores).  Connected 
with  the  reproductive  organs  are  branched  'fat  bodies'  which 
probably  are  connected  with  the  nutrition  of  the  reproductive 
structures  (p.  200). 

Fertilization  by  means  of  the  spermatophores  is  internal  in 
urodeles,  external  in  the  anura.  The  eggs  are  laid  in  the  water, 
and  left  without  further  care  by  most  forms.  A  few,  however, 
have  interesting  breeding  habits.  Thus  Amphiuma  and  IchtJiy- 
ophis  wrap  the  cords  of  eggs  around  the  body  ;  in  Alytcs  the 
male  wraps  the  cords  around  his  legs.  In  Rhinoderma  there  is 
a  large  gular  fold  into  which  the  eggs  are  received,  while  irr 
Nototrema  and  Notodclphys  a  brood  pouch,  open  behind,  is  formed 
by  a  duplication  of  the  skin  of  the  back.  In  the  Surinam  toad, 
Pipa,  the  eggs  are  spread  upon  the  back,  the  skin  of  which 
thickens  around  each  egg  so  that  it  assumes  the  character  of 

Honeycomb,  each  cell  being  occupied  by  an  egg  which  devel- 
•  As  in  this  position  until  the  adult  characters  are  assumed.  A 

P^^P  species  {Salamandra  atra,  S.  macnlosa,  Ccecilia  compressi- 
cauda)  bring  forth  living  young,  while  Amblystoma  tigrinum  fre- 
quently breeds  in  the  larval  or  '  Siredon  '  stage. 

The  eggs  contain  a  large  amount  of  yolk,  and  undergo  a 
total  but  unequal  segmentation  (Fig.  214),  the  result  being  the 
formation  of  a  blast ula  with  small  cells  on  one  side  and  larger 
(entodermic)  cells  on  the  other,  and  an  eccentric  segmentation 
cavity  (Fig.  215).  The  gastrula  arises  in  part  as  an  inpushing, 
in  part  as  the  result  of  an  overgrowth  of  the  ectoderm,  and  be- 


.-282 


CLASSIFICATION  OF   VERTEBRATES. 


fore  this  process  is  completed,  the  differentiation  of  the  central 
nervous  system  begins.  The  medullary  plate  infolds  into  a 
tube,  and  at  the  same  the  egg  begins  to  elongate  into  the  em- 
bryo. The  head  now  becomes  differentiated,  and  the  outlines 
of  the  eyes  are  seen,  while  the  tail  begins 
to  extend  behind,  the  ventral  surface  of 
the  embryo  being  swollen  by  the  large 
amount  of  yolk.  On  the  sides  of  the  neck 
appear  small  swellings,  the  rudiments  of 
the  external  gills,  two  pairs  in  the  anura, 
three  in  the  urodeles  and  some  caecilians. 
Besides  these,  the  anuran  develops  a  pair 
of  suckers  beneath  the  head,  while  the  uro- 
dele  is  characterized  by  the  formation  of 

c.  J 

a  pair  of  slender  rod-like  '  balancers'  in  front 
of  the  external  gills,  these  balancers  being 
apparently  the  gills  of  the  hyoid  arch. 
After  escape  from  the  egg  into  the  water 
the  gill  clefts  break  through.  The  limbs 
make  their  appearance  later  than  the 
external  gills. 

In  most  amphibia  there  is  a  metamor- 
phosis, most  marked  in  the  anura  where 
there  is  a  tailed  larva,  the  tadpole,  with 
small  toothless  mouth.  The  external  gills 
disappear  ;  the  tail  is  absorbed,  its  vertebrae 
being  reduced  to  the  urostyle  ;  the  internal 
gills  appear,  and  the  gill  slits  first  become 
enclosed  in  a  gill  chamber  formed  by  the 
backward  growth  of  the  opercular  fold,  and 
then  close  up  completely.  The  mouth  en- 
larges, and  the  tadpole  assumes  the  adult  form. 

All  the  facts  of  structure  and  development  go  to  show  that 
the  amphibia  have  arisen  from  the  crossopterygian  ganoids,  and 
that  existing  groups  have  descended  from  the  stegocephali, 
each  by  its  own  line  of  ancestry.  The  view  that  the  anura  have 
•descended  from  urodeles  has  little  morphological  evidence  in  its 
favor,  while  there  is  much  against  it. 


TIG.  281.     Larva  of 

-Amblystoma  punctata, 
enlarged,  showing  the 
.balancers. 


AMPHIBIA.  283 

SUB-CLASS   I.      STEGOCEPHALI 
(LAB  YRINTHODONTIA) . 

Extinct  amphibia  with  well-developed  tail ;  skull  solid,  with 
numerous  dermal  bones,  including  paired  supraoccipitals,  supra- 
temporals,  and  postorbitals  ;  the  lower  surface  of  the  body 
usually  with  three  large  ventral  bony  shields,  and  frequently 
with  smaller  scales  which  may  extend  over  the  dorsal  surface 
and  limbs  ;  a  separate  pubic  ossification.  The  stegocephali  ap- 
pear in  the  carboniferous  l  and  became  extinct  in  the  triassic. 
Some  were  of  gigantic  size,  and  in  some  the  dentine  of  the 
teeth  was  so  folded  as  to  give  these  animals  the  name  of  laby- 
rinthodonts. 

ORDER    I.     LEPOSPONDYLI. 

With  vertebral  centra  consisting  of  bony  envelopes  surround- 
ing the  persistent  notochord ;  teeth  simple,  with  large  pulp 
cavities.  Branchiosaurus  (one  species  about  four  feet  long)  had 
persistent  gills,  and  the  ventral  surface  of  body,  limbs,  and 
tail  with  oval  scales.  European  carboniferous.  Melanerpeton. 
The  MICROSAURIA,  with  pointed  heads  and  weak  fore  limbs,  are 
well  represented  in  the  carboniferous  of  Nova  Scotia  {Hylerpe- 
ton,  Hylonomus)  and  Ohio  (Tutidanus,  Colosteus)>  as  well  as  of 
Europe  (Keraterpetori).  In  the  AISTOPODA  the  body  was  snake- 
like  and  limbless.  DolicJiosoma,  Ophiderpeton,  European  car- 
boniferous ;  Phlegethontia  (coal  of  Ohio)  lacked  ribs. 

ORDER     II.    TEMNOSPONDYLI. 

Vertebrae  embolomerous  or  rhachitornous,  dentine  of  teeth 
radially  folded.  RHACHITOMI,  with  rhachitornous  vertebrae. 
Archcgosaurus,  the  best  known  stegocephalan  (European  car- 
boniferous), five  feet  long.  Trimerorhachis  (Texas  Permian)  had 
a  skull  five  feet  long.  Eryops  from  the  same  beds  was  half  as 
large.  EMBOLOMERI,  embolomerous  vertebrae.  Crtcotus,  Permian 
of  Texas  and  Illinois. 

1  Foot-prints,  possibly  of  a  stegocephalan,  have  recently  been  found  in  the  Devonian 
of  Pennsylvania. 


284 


CLASSIFICATION  OF   VERTEBRATES. 


ORDER   III.     STEREOSPONDYLI. 

Vertebrae  amphicoelous,  occipital  region  ossified,  teeth 
labyrinthine.  GASTROLEPIDOTI.  With  ventral  elongate  scales. 
Baphetes  (coal,  Nova  Scotia),  Platyops  (Permian  of  Europe). 
LABYRINTHODONTID^E,  no  ventral  scales.  Trematosaurus,  Laby- 
rint/wdon,  Mastodonsaurus,  etc.,  Europe. 


Jfa. 


FIG.  282.  Skull  of  stegocephal,  Trematosaurus,irom.  Huxley.  EpO,  epotic; 
Fr>  frontal ;  Ju,  jugal ;  La,  lachrymal;  Mn,  mandible  (of  several  bones);  Mx^ 
maxilla;  Na,  nasal ;  Or,  orbit;  Pa,  parietal;  Pmx,  premaxilla;  Prf,  prefrontal; 
Ptf,  postf  rental;  PtO,  postorbital;  Qt\  quadrate  jugal ;  SO,  supraoccipital ;  Sq, 
squamosal ;  St,  supratemporal.  The  grooves  shown  were  for  lateral  line  organs. 

SUB-CLASS   II.     URODELA.      (GRADIENTIA.) 

Amphibia  with  persistent  tails  ;  usually  two  pairs  of  limbs  ; 
skull  without  ethmoid,  supraoccipital,  postorbital,  or  supra- 
temporal  ;  no  parietal  foramen.  Vertebrae  amphicoelous,  never 
embolomerous  or  rhachitomous.  Skin  naked. 

ORDER  I.  PERENNTBRANCHIATA  (PHANEROBRANCHIA). 

With  persistent,  bushy,  external  gills  and  gill  slits  ;  maxilla 
usually  lacking;  teeth  on  vomers  and  palatines.  SIRENID^E,  hind 


AMPHIBIA.  285 

limbs  lacking  ;  Siren,  the  mud  eel  of  southern  United  States,  has 
jaws  armed  with  horny  sheaths.  PROTEID.E,  hind  limbs  present ; 
jaws  with  teeth.  Proteus  of  Austrian  caves  nearly  blind ; 
Necturus  (Menobranchus),  the  mud  puppy  of  the  central  United 
States. 

ORDER   II.     DEROTREMATA   (CRYPTOBRANCHIA). 

External  gills  lost,  a  spiracle  on  the  side  of  the  neck,  lead- 
ing to  persistent  gill  slits.  AMPHIUMID.E,  limbs  rudimentary  ; 
Amphiuma,  one  species,  the  congo  eel  from  the  southern  states. 
CRYPTOBRANCHID^:,  legs  strong;  body  salamander-like.  Men- 
opoma  (Cryptobranchus),  hell-bender,  from  U.  S.  Megalobatra- 
chus,  giant  salamander  from  Japan,  three  feet  long.  Andrias 
scheuchzeri,  European  miocene,  described  over  one  hundred  and 
fifty  years  ago  as  a  relic  of  the  legendary  Noachian  deluge. 

ORDER   III.     SALAMANDRINA   (MYCTODERA). 

Gill  slits  and  external  gills  lost  in  the  adult  ;  vertebrae  fully 
ossified.  LECHRIODONTA,  palatine  teeth  in  a  transverse  row  or 
posteriorly  converging  series.  Amblystoma,  toothless  parasphen- 
oid,  toes  four  in  front,  five  behind ;  many  species  in  U.  S. 
PletJwdon,  teeth  on  parasphenoid ;  premaxillaries  separate. 
Spelerpes,  premaxillaries  fused ;  Dcsmognathust  with  parasphen- 


FIG.  283.     Plethodon  erythronotus. 

oid  teeth  and  opisthoccele  vertebrae.  The  species  of  Amblys- 
toma  are  remarkable  for  the  length  of  time  that  their  larvae 
(Siredori)  retain  their  gills,  some  species  (A.  tigrinum)  and  the 
axolotl  of  Mexico  breeding  in  the  siredon  stage.  Most  of  the 
lungless  salamanders  (p.  27)  belong  in  this  family.  MECOD- 
ONTA,  parasphenoid  toothless,  palatine  teeth  in  two  rows  diver- 
ging behind.  Diemyctylus,  our  common  newt.  In  Europe  Triton, 


286  CLASSIFICATION  OF   VERTEBRATES. 

Salamandra,  Pleurodeles,  etc.,  the  first  two  genera  dating  from 
the  European  miocene.     Megalotriton,  eocene. 


FIG.  284.     Siredon  lava  of  Atnblystoma^  from  Hertwig,  after  Dumeril  and  Bibron. 

SUB-CLASS   III.     ANURA    (SALIENTIA). 

Tailless  in  the  adult  condition,  the  caudal  vertebrae  being 
reduced  and  fused  to  a  urostyle  ;  vertebrae  usually  procoelous  ; 
frontoparietals  fused  ;  sphenethmoid  present  ;  hind  legs  elongate 
and  fitted  for  leaping,  the  proximal  row  of  tarsals  greatly  elon- 
gate ;  a  marked  metamorphosis,  the  tadpoles  being  vegetarians, 
the  adults  carnivorous.  The  anura  contains  the  frogs,  toads, 
tree-toads,  etc.,  the  group  being  best  developed  in  North  Amer- 
ica and  in  the  tropics.  Its  origin  is  uncertain,  but  probably  was 
from  some  stegocephalian  ancestor. 

ORDER   I.     AGLOSSA. 

Tongue  lacking  ;  the  Eustachian  tubes  open  together  into 
the  pharynx ;  epicoracoids  free,  but  not  overlapping.  Xenopus 
{Dactylethrd),  from  Africa;  Pipa,  the  Surinam  toad  (p.  281), 
from  South  America. 

ORDER   II.     ARCIFERA. 

Tongue  well  developed  ;  shoulder  girdle  arciferous  (p.  278), 
the  coracoids  of  the  two  sides  overlapping ;  Eustachian  tubes 
widely  separate.  The  BUFONID^:  includes  the  toads,  in  which 
the  jaws  are  toothless,  the  toes  webbed,  but  without  suckers  at 
the  tips  ;  parotid  glands  prominent.  Bufo,  the  common  toad. 
The  genus  appears  in  the  eocene.  The  PELOBATID.E  differ  in 
having  teeth.  Pelobates  first  appears  in  the  miocene.  Scapin- 


AMPHIBIA. 


287 


opus  includes  the  burrowing  spade-foot  toad  which  is  rarely 
seen  except  at  the  breeding-season.  Allied  European  genera 
are  Alytes  and  Bombinator.  The 
HYLID.E  have  teeth,  while  the 
tips  of  the  toes  are  expanded  into 
sucking-disks.  Our  tree-toads  be- 
long to  Hyla,  Acris,  and  Choro- 
philus ;  Notodelphys  and  Nototrema, 
tropital  America.  The  extinct 
PAL^OBATRACHID^:  (oligocene) 
are  noticeable  for  two  sacral  ver- 
tebrae. 


ORDER   III.     FIRMISTERNIA. 


FIG.  285.  Shoulder  girdle  of 
Bombinator  igneus,  showing  the  ar- 
ciferous  type,  after  Wiedersheim. 
f,  clavicle  ;  c0,  coracoid ;  ec,  epi- 
coracoid  ;  g,  glenoid  fossa  ;  /<:,  pro- 

Tongue    well     developed;     epi-  corac<?id;  ''  scapula;   JJ'  supra- 

scapula  ;  sf,  sternum. 

coracoids     firmly     united     in    the 

median  line.  The  ENGYSTOMID.E,  or  toothless  frogs,  occur  in 

our  southern  states.  Engy- 
stoma.  The  RANID.E,  or 
true  frogs,  have  smooth 
skin,  and  teeth  in  the  up- 
per jaw.  Rana  contains 


our  species  including  the; 
bullfrog     (A*,     catesbianay 
the    largest    known    frog. 
Sternum    and  Rana  first  appears  in  the 


the    Boulder 


j  Numerous  other 


after   Wieders- 

the    firmister-  families  in  the  tropics,  in- 


FIG.  286. 
ventral  portion  of 
girdle  of  Rana, 
heim,  illustrating 

nous  type  of  sternum,     cl,  clavicle ;    co,  cora-    eluding       the     DENDROBA- 
coid;    ec;    epicoracoid ;    g,  glenoid  fossa ;    as,    TIDJE    which  have  tOCS  like 
omosternum;    s,  ventral  part  of  scapula;    st, 
sternum;  x,  xiphisternum.  tne  tree-toads,  Hylldae. 

SUB-CLASS    IV.     GYMNOPHIONA  (CffiCILLffi). 

Limbless  amphibia  of  worm-like  shape  ;  tail  lacking;  vertebrae 
amphicoelous  ;  skull  well  ossified,  with  well-developed  ethmoid  ;. 
body  externally  ringed,  and  bearing  semi-circular  dermal  scales. 
Frequently  a  protrusible  tentacle  in  a  tentacular  sheath  between 


288 


CLASSIFICATION  OF   VERTEBRATES. 


the  orbit  and  nostril.  The  caecilians  are  tropical,  occuring  in 
South  America,  Africa,  and  Ceylon,  where  they  burrow  in  the 
earth,  preying  upon  small  invertebrates. 
The  eyes,  in  consequence  of  this  life,  are 
hidden  under  the  skin.  Little  is  known 
of  the  development,  except  of  the  Cey- 
lonese  species,  Ichthyophis  glutinosus,  in 
which  the  larva  has  three  pairs  of  pectin- 
ate external  gills.  In  the  larval  TypJiIo- 
nectcs  the  gills  are  saccular.  Other  genera 
are  C&cilia,  Rhinotrema,  and  Hypogeophis. 
No  fossil  species  are  known,  but  the  dis- 
tribution as  well  as  the  characters  of  the 
skeleton  point  to  a  great  ancestry  for  the 
group.  Within  recent  years  it  has  been 
supposed  to  be  related  to  Amphinma,  but 
this  is  clearly  not  the  case.  The  aisto- 
poda  (p.  283)  suggest  themselves  in  this 
connection. 


FIG.  287.  Tentacle  of 
Cecilia  oxyura,  after 
\Viedersheim.  do,  duct 
of  orbital  gland;  <//,  duct 
of  tentacular  gland;  ,,  GRADE  IL  AMNIOTA  (ALLAN- 

TOIDEA). 


eye;  m,  mouth  of  ten- 
tacle; ng,  nasal  gland; 
og,  orbital  gland;  r,  re- 
tractor of  tentactle;  tg, 
tentacular  gland. 


Vertebrates  with  well-developed  amnion 
and  allantois  ;  no  gills,  no  lateral  line  sys- 
tem, and  no  rental  portal  system  in  the  adult. 
The  amniotes  derive  their  name  from  the  existence  during 
fcetal  life  of  a  peculiar  envelope  —  the  amnion.  This  consists  of 
folds  of  the  somatopleure  (head,  tail,  and  lateral  folds)  which 
grow  upwards  on  all  sides  of  the  embryo,  meeting  and  fusing 
above  the  back,  so  that  the  embryo  is  enclosed  in  a  cavity 
bounded  by  double  walls,  that  nearest  the  embryo  being  the 
amnion,  the  other  being  the  chorion.  The  amnotic  cavity  is 
filled  with  an  amniotic  fluid.  Both  amnion  and  chorion  are 
composed  of  ectoderm  and  the  somatic  mesothelium  of  the  lateral 
plates,  and  the  space  between  them  is  an  extension  of  the 
ccelom.  With  growth,  the  amniotic  structures  become  connected 
with  the  embryo  by  only  a  small  stalk,  the  umbilicus,  on  the 


AMNIOTES. 


289 


ventral  side,  through  which,  as  will  be  seen  from  Fig.  288,  D, 
pass  the  stalks  of  the  yolk  sac  and  of  the  allantois  next  to  be 
mentioned. 


FIG.  288.  Diagram  of  the  foetal  envelopes  of  an  amniote.  A,  before  the  union 
of  the  amniotic  folds  ;  £,  transverse  section  of  A ;  6",  union  of  amniotic  folds  ; 
D,  outgrowth  of  allantois  and  reduction  of  yolk  sac.  a,  amnion  ;  a/,  allantois  ;  am, 
amniotic  folds;  ac,  cavity  of  amnion  ;  c,  ccelom;  z,  alimentary  tract;  s,  serosa ;  so, 
somatopleure ;  sp,  splanchnopleure  ;  y,  yolk  sac ;  ys,  yolk  stalk.  The  somatic  meso- 
derm  by  dashes,  the  splanchnic  layer  dotted ;  ectoderm  and  entoderm  a  continuous 
line. 

The  allantois  is  represented  in  the  amphibia  by  a  ventral 
outgrowth  from  the  hinder  portion  of  the  alimentary  canal, 
which  never  extends  beyond  the  body  walls,  but  develops  in 
situ  into  the  urinary  bladder.  In  the  amniotes,  on  the  other 
hand,  this  outgrowth  is  more  extensive,  and  extends  outward  be- 


CLASSIFICATION  OF   VERTEBRATES. 

hind  the  yolk  stalk  into  the  extra  embryonic  part  of  the  coelom 
(Fig.  288),  carrying  with  it  the  allantoic  artery  and  the  umbilical 
vein  or  veins.  Distally  it  expands  into  >  a  large  sac  (which  re- 
ceives the  excretion  of  the  primitive  kidneys),  the  outer  surface 
of  the  sac  fusing  with  the  chorion.  The  result  of  this,  in  ovipa- 
rous forms,  is  that  the  allantoic  structures  come  to  lie  close 


FIG.  289.  Diagram  of  embryonic  circulation  in  an  amniote,  the  amnion 
omitted  for  clearness.  A,  allantois  ;  A  A,  allantoic  artery;  C,  carotids  ;  CA,  caudal 
artery;  CV,  caudal  vein;  DA,  dorsal  aorta  ;  DC,  ductus  Curvierii;  H,  heart;  HA, 
hypogastric  artery;  L,  liver;  OA,  OR,  omphalomesaraic  artery  and  vein;  UV, 
umbilical  vein  ;  V,  vent;  VV,  vitelline  vein.  The  outline  of  the  alimentary  canal 
blocked. 

beneath  the  shell,  and  hence,  with  their  rich  blood-supply,  they 
form  efficient  organs  of  foetal  respiration.  In  the  higher  mam- 
mals this  allantois  enters  into  close  connection  with  the  uterine 
walls,  thus  giving  rise  to  a  structure  both  nutrient  and  indirectly 
respiratory  in  character,  the  placenta,  the  features  of  which  will 
be  described  in  connection  with  that  group. 

Basi-  and  presphenoid  bones  are  present,  and  a  parasphenoid 
occurs  only  in  some  reptiles  (ophidia)  as  a  small  plate.  The 


SAUROPSIDA, 


29I 


ribs  are  developed  in  connection  with  the  transverse  processes, 
and  the  skeleton  of  the  limbs  (when  present)  is  reducible  to  the 
pentadactyl  type.     A  sternum  is  present  except  in  some  apodal 
forms,  and  is  developed  in  connection 
with  the  ribs  ;  and  the  branchial  arches 
are  much  reduced  and  modified. 

Gill  pouches  occur,  and  some  of 
these  may  break  through  to  the  ex- 
terior; but  in  no  case  are  gills  devel- 
oped in  connection  with  them,  and 
they  never  serve  in  connection  with 
respiration.  The  alimentary  canal 
either  terminates  in  a  cloaca,  or  the 
vent  is  behind  the  urogenital  open- 
ings. The  heart  always  has  two  au- 
ricles, the  sinus  venosus  becoming  FlG  2QO  Human  embryo 
included  in  the  right  of  these,  while  with  the  floor  of  the  mouth 
the  ventricle,  either  partially  or  com-  removed,  after  Hertwig.  b, 

,        ,        ,.    .  ,     ,    .  ,          .       ,.       ,  branchial     clefts:     s,     cervical 

pletely  divided  by  a  longitudinal  sep-    sinus;  ^  eye;  ^  hypophysiai 
turn,  is  at  least  physiologically  divided    pocket;  /,  lungs;  n,  nostril, 
into  arterial  and  venous  halves. 

In  the  adult  true  kidneys  (metanephros)  are  developed,  the 
renal  portal  system  is  reduced  or  lacking  in  the  higher  forms, 
and  the  posterior  cardinals  become  greatly  reduced  (p.  196). 

The  amniotes  are  divided  into  the  Sauropsida  and  the 
Mammalia. 

CLASS  I.     SAUROPSIDA  (MONOCONDYLIA). 

Amniote  vertebrates  with  one  occipital  condyle ;  lower  jaw 
suspended  by  the  free  or  fixed  quadrate;  ankle  joint  between 
the  first  and  second  rows  of  carpals  or  tarsals  ;  coracoid  well 
developed  ;  external  surface  covered,  at  least  in  part,  with  ecto- 
dermic  scales  ;  corpus  callosum  rudimentary  ;  heart  three  or  four 
chambered,  red  blood  corpuscles  small,  oval,  nucleated  ;  a  cloaca 
present ;  the  eggs  are  large,  and  undergo  a  partial  (meroblastic) 
segmentation  ;  all  except  a  few  forms  are  oviparous,  and  the 
eggs  are  enclosed  in  a  more  or  less  calcareous  shell. 

Besides  the  features  of  the  diagnosis,  several  other  points  are 


CLASSIFICATION  OF   VERTEBRATES. 

characteristic  of  the  sauropsida.  The  skin  is  remarkably  defi- 
cient in  glands,  these,  when  present,  usually  occurring  upon  the 
legs  or  upon  the  tail.  The  characteristic  scales  are  cornifica- 
tions  of  the  epidermis,  and  are  occasionally  re-enforced  by  bony 
plates  developed  in  the  dermis.  The  single  occipital  condyle  is 
situated  on  the  basioccipital,  the  exoccipitals  contributing  to  its 


pao. 


FIG.  291.  Base  of  skull  of  alligator,  showing  the  single  occipital  condyle.  bo, 
basioccipital  ;  l>s,  basisphenoid ;  eo,  exoccipital ;  et,  opening  of  Eustachian  tube ;  fmt 
foramen  magnum ;  pao,  paroccipital ;  //,  pterygoid  ;  q,  quadrate  ;  qj,.  quadratojugal ; 
sq,  squamosal ;  tr,  transversum. 

formation  to  a  varying  extent.  The  mandible  consists  of  a  single 
cartilage  bone,  the  articulare,  and  at  most  five  membrane  bones, 
—  dentary,  splenial,  coronoid,  angulare,  and  surangulare.  The 
cervical  ribs  are  usually  well  developed,  the  neck  passing  insen- 
sibly into  the  thorax.  The  ovarian  ducts  have  their  inner  ends 
entire  as  in  the  ichthyopsida. 

The  sauropsida  contains  the  Reptilia  and  the  Aves. 

SUB-CLASS   I.     REPTILIA. 

Cold-blooded  amniotes ;  the  external  surface  of  the  body 
(except  in  a  few  fossil  forms)  covered  with  horny  epidermal, 
scales  or  bony  dermal  plates  ;  anterior  appendages,  when  present, 
ambulatory  (except  in  pterodactyls),  the  carpals  and  meta- 
carpals  numerous  ;  sacral  vertebrae  usually  two  ;  pubic  and  is- 
chiadic  bones  united  by  symphysis,  except  in  some  dinosaurs  ; 
persisting  right  and  left  aortic  arches. 


REPTILES.  293 

The  living  reptiles  in  their  external  form  present  three 
types:  (i),  the  quadrupedal  long-tailed  form  represented  by  the 
lizards  and  alligators  ;  (2),  the  cuirassed  forms  of  the  turtles  ; 
and  (3),  the  apodal  forms  of  the  snakes  and  footless  lizards. 
If  the  fossil  groups  also  be  taken  into  consideration  the  range 
of  shape  is  still  greater  ;  for  it  includes  not  only  the  swimming- 
groups,  the  plesiosaurs  and  ichthyosaurus,  but  the  flying  reptiles, 
the  pterodactyls. 

A  few  of  the  fossil  forms  apparently  had  naked  skins  ;  but 
in  the  rest  the  body  is  more  or  less  completely  covered  by 
scales,  which  differ  from  those  of  the  ichthyopsida,  in  that  they 
are  cornifications  of  the  superficial  layers  of  the  epidermis. 
These  are  re-enforced  in  many  by  dermal  ossifications,  which  may 
be  minute  as  in  certain  lizards,  or  larger  scutes,  as  in  the  croco- 
diles and  in  many  extinct  groups ;  whereas  in  some  fossil  croc- 
odiles (Teleosaurus)  and  dinosaurs,  they  form  a  complete  armor 
for  the  body.  In  the  turtles  this  formation  of  armor  reaches 
its  extreme,  for  here  the  dermal 
plates  are  usually  united  with  the 
ribs  to  form  a  firm  carapace  and 
plastron.  Usually  there  is  no 
pigment  in  the  epidermis  ;  but  the 
derma  contains  pigment  cells, 
which  in  certain  lizards  (Anolis, 
Chameleo)  are  capable  of  proclu-  F,c.  292.  Section  and  medial  view 

of  jaw  of  Anotts,  showing  pleurodont 

cmg  marked  color  changes  under   dentition. 

control    of    the    nervous    system. 

Epidermal  glands  are  rare.      In  some  turtles  scent  glands  occur 

beneath  the  mandibles  or  on  the  side  of  the  plastron  ;   in  the 

snakes   and   crocodiles   similar  glands    are   connected  with   the 

cloaca ;    while    in    most    lizards    there    is    a    row  of   glands   on 

the  ventral  surface  of  the  femur. 

Teeth  (lacking  in  turtles  and  some  .pterodactyls  and  anomo- 
dontia)  are  usually  restricted  to  the  premaxillary,  maxillary,  and 
dentary  bones  ;  but  in  snakes  and  some  lizards  they  may  also 
occur  upon  the  palatines  and  pterygoids.  These  teeth  are 
usually  simple,  without  folding  of  enamel,  and  only  in  the  therio- 
dontia  are  they  differentiated  into  incisors,  canines,  and  molars. 


294  CLASSIFICATION  OF    VERTEBRATES. 

In  the  majority  of  reptiles  the  teeth  are  either  anchylosed  to 
the  edge  of  the  jaws  (acrodont),  or  by  their  sides  to  the  wall 
of  a  groove  (pleurodont) ,  while  in  crocodiles  and  many  dinosaurs 
they  are  implanted  in  sockets  or  alveoli  (thecadont)  ;  usually 
the  teeth  are  in  a  single  row.  In  the  snakes  the  teeth  are 
grooved,  and  in  the  poisonous  species  the  grooves  in  one  pair 
may  be  very  deep  or  completely  converted  into  a  canal,  which 
is  to  convey  the  poison  into  the  wound  made  by  these  fangs. 
As  a  rule  the  teeth  are  used  for  the  prehension  of  the  prey, 
and  only  in  the  herbivorous  orthopoda  are  they  of  value  in 
the  comminution  of  food.  In  the  turtles,  and  apparently  in  the 
extinct  edentulous  forms,  the  jaws  are  covered  with  an  epider- 
mal horny  beak. 

Salivary  glands  are  lacking  in  the  marine  chelonians  and  in 
the  alligator,  while  in  the  crocodiles  they  occur  only  on  the 
tongue.  In  other  reptiles  lingual,  sub-lingual,  palatine,  and 
labial  glands  may  occur,  the  poison  glands  of  ophidians  being 
modified  labials.  The  tongue  is  either  spatulate  and  immobile, 
as  in  crocodiles,  turtles,  and  a  few  lizards,  or  bifid  and  exten- 
sile in  other  forms  ;  its  variations  of  shape  being  of  value  in 
the  classification  of  the  lacertilia. 

In  the  alimentary  canal  the  most  noticeable  features  are 
the  wide  oesophagus,  correlated  with  the  swallowing  of  the  food 
entire,  and  the  large  intestine,  frequently  provided  with  a  caecum 
near  the  ileo-colic  valve.  In  the  turtles  the  oesophagus  is 
armed  with  numerous  papillae  pointing  backward.  The  liver 
is  usually  bilobed,  but  in  the  snakes  and  snake-like  amphisbae- 
nians  it  is  unilobular  and  elongate. 

At  no  time  is  there  a  branchial  respiration,  the  lungs  being 
the  sole  organs  of  exchange  of  gases.1  The  glottis  is  supported 
by  well-developed  cricoids  and  arytenoids  ;  the  trachea  is  long, 
and  in  crocodiles  and  turtles  may  be  bent  into  a  loop.  The 
tracheal  and  bronchial  rings  are  better  developed  than  in  the 
amphibia.  The  lungs  show  variations  in  shape  and  size ;  and 
in  the  elongate  reptiles  the  left  lung  is  the  smaller,  and  may 
even  be  reduced  to  a  rudiment  (snakes).  In  these  forms  the 

1  Experiments  go  to  show  that  the  pharyngeal  epithelium  of  certain  North  American 
and  Australian  turtles  has  a  respiratory  function. 


REPTILES.  295 

posterior  dorsal  portion  or  the  right  lung  is  supplied  with  blood 
from  the  dorsal  aorta.  In  the  chameleons  and  geckoes  the 
lungs  give  off  large  saccular  projections,  recalling  the  air  sacs 
so  characteristic  of  birds.  In  some  dinosaurs  the  bones  exhibit 
a  marked  pneumaticity,  and  it  is- supposed  that  in  these  the  air 
sacs  penetrated  the  bones.  In  the  snakes  the  chambering  of 
the  lung  is  restricted  to  the  peripheral  portion,  the  centre  being 
occupied  by  a  large  air  space,  and  about  the  same  conditions 
occur  in  most  lizards.  In  the  chameleons,  however,  each  bron- 
chus, on  entering  the  lungs,  divides  into  three  parts,  and  the 
proximal  portion  of  the  lung  is  sacculated,  while  distally  all  three 
bronchi  connect  with  a  common  space,  without  alveoli.  In 
crocodiles  and  chelonians  the  sub-division  of  the  lungs  is  carried 
farther. 

The  brain  presents  advances  in  several  points  upon  the  con- 
ditions in  the  amphibia.     Thus  there  is  here  developed  a  cere- 
bral cortex  of  gray  matter  containing  pyramidal 
cells.     The  cerebrum  exhibits  a  tendency  to  ex- 
tend backwards,  covering  in  the  thalamencepha- 
lon.    The  olfactory  lobes  may  be  seated  directly 
on  the  cerebrum,  or  an  elongate  olfactory  tract 
may  intervene.     The  olfactory  fibres  do  not  ex- 
tend back  to  the  corpus  striatum,  but  a  distinct      / 
olfactory   centre   is   developed   in    the    pallium.     /  . 
Hippocampal  lobes  occur  in  a  few  forms  {Hat-     M 
teria,  crocodiles,  chelonians).      The  twixt  brain 
is  at  a  lower  level  than  the  rest,  the  infundib- 
ular   region    being  well    developed.      The    mid 
brain  is  large,  and  its  two  halves  rarely  exhibit 
a  tendency  towards   division  into  four.      In  the 
cerebellum  there  is  a  great  range  of  structure, 
from  forms  in  which  it  is  merely  a  transverse  i 

fold,  up  to  the  crocodiles,  where  it   consists  of     Brain   of   garter- 
two  lateral  lobes  and  a  median   portion,  recall-    snake,    Eutainia 
ing  the  vermis  of  the  mammals.    In  the  medulla 
occurs  the  characteristic  nuchal  flexure. 

In  the  cranial  nerves  the  marked  feature  is  the  distinct  ori- 
gin of  nerves,  the  roots  of  which  are  closely  approximate  in  the 


296  CLASSIFICATION  OF   VERTEBRATES. 

amphibia ;  thus  the  facialis  is  distinct  from  the  trigeminal ;  the 
eye-muscle  nerves  have  distinct  roots  ;  the  glossopharyngeal  is 
distinct  from  the  vagus  ;  the  accessorius  is  a  distinct  nerve, 
except  in  ophidia,  and  the  hypoglossal  becomes  a  cranial  nerve, 
passing  through  a  foramen  in  the  cranial  wall. 

The  nostrils  are  usually  terminal,  but  are  just  in  front  of  the 
orbits  in  ichthyosaurs  and  plesiosaurs.  In  the  lizards  the  nasal 
passage  is  divided  into  an  anterior  vestibule  and  a  posterior 
olfactory  region,  and  in  these,  as  in  the  ophidia,  the  amount  of 
olfactory  surface  is  increased  by  the  presence  of  a  turbinal  bone. 
In  the  turtles,  and  still  more  in  the  crocodiles,  the  nasal  pas- 
sage is  divided  horizontally  into  an  upper  olfactory  and  a  lower 
respiratory  tract.  Glands  occur  in  connection  with  the  nose 
in  most  reptiles,  while  in  all  except  crocodiles  and  turtles  an 
organ  of  Jacobson  occurs. 

The  eyeball  is  nearly  spherical ;  the  sclerotic  which  sur- 
rounds it  is  cartilaginous,  and  in  it  are  frequently  developed  (as 
in  many  birds)  a  ring  of  bony  sclerotic  plates.  A  tapetum  is 
developed  in  the  lacertilia,  but  the  argentea,  so  characteristic  of 
lower  vertebrates,  is  lacking.  In  many  there  is  an  internal 
structure,  the  pecten,  homologous  with  the  process  falciformis 
of  the  fishes.  Eyelids  are  usually  present,  the  third  being  fre- 
quently developed.  In  snakes  and  some  lizards  the  lids  are 
transparent,  and  their  edges  are  united  together  so  that  a  lachry- 
mal space  is  enclosed  between  them  and  the  conjunctiva.  In 
many  lizards  and  in  Hatteria  the  parietal  eye  (Figs.  90  and  92) 
is  extremely  well  developed,  and  is  situated  in  a  foramen  in  the 
roof  of  the  skull.  Many  fossil  reptiles  belonging  to  different 
orders  have  a  similar  parietal  foramen,  thus  suggesting  the  for- 
mer presence  of  a  visual  organ  in  these  forms. 

In  the  inner  ear  the  lagena  is  large,  and  in  the  crocodiles 
shows  the  beginnings  of  a  spiral  coiling,  recalling  the  cochlea  of 
the  mammals.  With  its  increase  in  length  the  macula  lagenae 
is  correspondingly  elongated.  The  middle  ear  and  Eustachian 
tube  are  lacking  in  adult  snakes  and  amphisbaenans,  the  colu- 
mella  auris  in  these  forms  being  embedded  in  the  flesh.  The 
stapes  is  continuous  with  the  columella,  and  in  rhynchocephalia 
the  columella  is  connected  with  the  hyoid.  In  many  lizards  and 


REPTILES.  297 

chelonians  the  tympanic  membrane  is  exposed  ;  in  some  lizards 
it  is  partially  covered  by  a  flap  developed  from  in  front,  while  in 
the  crocodiles  the  flap  is  movable  and  the  tympanum  is  some- 
what sunken,  the  beginnings  of  the  auditory  meatus  of  the 
higher  vertebrates. 

In  the  skeleton  the  ossifications  are  far  more  extensive  than 
*H  the  amphibia.  The  notochord  does  not  persist,  except  inter- 
vertebrally  in  a  few  forms  (geckoes  and  rhynchocephalia).  The 
vertebrae  are  usually  precocious  ;  but  amphicoelous  vertebrae  occur 
in  some  or  all  theromorpha,  ichthyosauria,  plesiosaurs,  rhyncho- 
cephalia, geckoes,  theropoda,  orthopoda,  and  ornithopoda,  while  in 
a  few  dinosaurs  they  are  flat  (amphiplatyan).  In  many  groups 
the  neural  arches  are  anchylosed  to  the  centra,  or  again,  as  in 
ichthyosaurs,  turtles,  and  crocodiles,  they  are  united  by  suture. 
Haemal  arches  occur  in  snakes,  lizards,  and  crocodiles.  Trans- 
verse processes,  when  present,  are  borne  on  the  neural  arch  (i.e., 
are  diapophyses). 

At  most  five  regions  can  be  distinguished  in  the  column  ;. 
but  in  the  snakes,  where  no  limbs  are  formed,  only  trunk  and 
caudal  vertebrae  can  be  distinguished.  In  the  plesiosaurs  axis- 
and  atlas  are  fused ;  the  proatlas  of  the  crocodilia  has  beert 
referred  to  (p.  143).  Usually  there  are  two  sacral  vertebrae. 

Ribs  are  usually  present,  and  may  be  either  with  a  single 
head  or  bicipital.  In  the  snakes  they  may  extend  the  whole 
length  of  the  trunk  with  the  exception  of  the  atlas.  In  the 
crocodilia  and  Hatteria  thoracic  and  abdominal  ribs  are  dis- 
tinguished, the  latter  developing  in  the  myocommata  of  the 
ventral  surface,  and  not  extending  to  the  vertebrae  (see  p.  147). 
Cervical  ribs  are  entirely  lacking  in  the  turtles,  while  in  the 
same  group  the  thoracic  ribs  are  united  to  the  dermal  plates 
forming  the  carapace. 

A  sternum  is  lacking  in  plesiosaurs,  ichthyosaurs,  turtles,, 
snakes,  and  some  snake-like  lizards,  while  there  is  little  evidence 
as  to  its  structure  in  the  theromorphs  and  dinosaurs,  where  it 
was  apparently  largely  cartilaginous.  When  present  it  is  tri- 
angular or  rhomboidal  in  outline,  and  contains  no  membrane 
bone.  In  the  flying  reptiles  (pterodactyls)  it  had  a  strong  ven- 
tral keel  for  attachment  of  the  wing  muscles.  The  episternum 


.298 


CLASSIFICATION  OF   VERTEBRATES. 


.(lacking  in  chameleons)  is  usually  well  developed,  and  affords  a 
support  for  the  ventral  ends  of  the  clavicles.     In  many  turtles 
it,  together  with  the  clavicles,  enters  into 
the  formation  of  the  plastron. 

The  skull  of  recent  reptiles  differs  in 
many  respects  from  that  of  existing  am- 
phibia ;  but  when  the  fossil  groups  are  con- 
sidered, the  distinctions  largely  disappear, 
the  skulls  of  stegocephalans  and  thero- 
morphs  being  strikingly  similar.  In  these 
lower  reptiles  the  top  of  the  skull  forms  a 
continuous  roof  above  the  attachment  of 
the  jaw  muscles  ;  but  in  other  groups  gaps 
or  vacuities  may  occur,  so  that  these  mus- 
cles are  exposed  from  above.  These  va- 
cuities or  fossae  exhibit  the  following  vari- 
ations :  (i),  between  the  parietals  and 
postorbitals  (supraternporal  fossa)  ;  (2), 
between  postorbitals  and  squamoso-jugal 
(infratemporal  fossa)  ;  (3),  between  the 
post-temporal  and  the  exoccipital  and  op- 
isthotic  (post-temporal  fossa)  ;  (4),  the 
line  of  bones  (arcade)  between  i  and  2 
may  be  interrupted,  producing  one  large 
temporal  fossa;  (5),  the  squamosojugal  arcade  may  be  discon- 
tinuous. 

As  a  rule  the  cartilage  of  the  primordial  cranium  is  largely 
replaced  by  bone,  the  ethmoid  and  parts  of  the  sphenoid  alone 
being  incompletely  ossified.  Except  in  a  few  theromorphs  there 
is  but  a  single  occipital  condyle,  which  is  either  formed  by  the 
basioccipital  alone  or  with  the  participation  of  the  exoccipitals. 
Either  basi-  or  supraoccipitals  may  be  excluded  from  the  for- 
amen magnum.  In  the  ear  region  a  fenestra  rotunda  appears  ; 
•of  the  otic  bones  the  prootic  is  always  distinct,  the  epiotic  is 
fused  with  the  supraoccipital,  while  the  opisthotics  (free  in 
turtles)  are  usually  united  to  the  exoccipitals. 

While  in  some  the  brain  extends  forwards  between  the  orbits, 
it  frequently  does  not  reach  so  far  forward,  and  the  orbits  them- 


FIG.  294.  Pectoral 
girdle  and  sternum  of 
lizard,  L&manctus  lon- 
gipes,  after  Parker,  c, 
coracoid  ;  cl,  clavicle ; 
.*?,  epis'ternum  ;  g,  glenoid 
iossa ;  /,  procoracoid  ; 
^',  rib ;  s,  scapula ;  st, 
sternum ;  x,  xiphister- 
num.  Cartilage  dotted. 


REPTILES.  299 

selves  are  separated  by  a  more  or  less  complete  interorbital  sep- 
tum. Correlated  with  this  is  the  frequent  absence  of  all-  and 
orbitosphenoid  ossifications,  their  places  being  taken  by  vertical 
processes  of  parietals  (turtles)  or  frontoparietals  (snakes) ; 
while  the  frontals  frequently  take  no  part  in  roofing  in  the  cra- 
nial cavity,  but  are  placed  above  the  interorbital  septum. 

The  membrane  bones  of  the  cranium  are  numerous,  the 
frontals  and  parietals  of  the  two  sides  being  frequently  fused  in 
the  median  line.  Between  the  parietals  in  Hatteria  and  most 


FIG.  295.  Lateral  view  of  the  skull  of  Hatteria  (Sphenodon},  after  Giinther. 
_//',  frontal ;  y,  jugal ;  /,  lachrymal;  true,  maxillare ;  n,  nasal;  oo,  opisthotic ;  pa, 
palatine ;  pf,  prefrontal ;  pm,  premaxillary ;  po,  postorbital ;  pof,  postfrontal  ;  //, 
pterygoid;  q,  quadrate;  </j,  quadratojugal ;  st/,  squamosal.  The  supra- and  infra- 
temporal  fossae  shown  above  and  below  the  postorbital-squamosal  arch. 

lizards,  as  well  as  in  many  fossil  forms,  occurs  a  well-marked  in- 
terparietal  foramen,  connected,  at  least  in  the  living  forms,  with 
a  well-developed  parietal  eye.  The  ethmoid  region  is  covered 
by  the  paired  nasals,  while  in  lizards  they  are  covered  by  paired 
vomers.  In  other  forms  the  vomer  may  be  median  and  un- 
paired. Prefrontals  are  almost  always  present,  while  postfrontals 
usually  occur ;  and  in  lizards,  crocodiles,  and  many  extinct  forms 
lachrymals  are  present.  In  many  fossils  and  in  lizards  a  supra- 
temporal  bone  occurs  between  squamosal  and  quadrate ;  while  in 
lizards,  snakes,  crocodiles,  and  ichthyosaurs  an  os  transversum 
connects  the  maxillary  with  the  pterygoid.  In  dinosaurs  a 
rostral  bone  may  occur  in  front  of  the  premaxilla. 

The  lower  jaw  is  always  suspended  from  the  quadrate;  and 


3oo 


CLASSIFICATION  OF   VERTEBRATES. 


this  bone  may  be  either  freely  movable  or  firmly  united  by 
suture  to  the  adjacent  bones,  the  first  condition  occurring  only 
in  snakes  and  lizards.1  The  two  rami  of  the  lower  jaw  are 
usually  united  by  ligament  or  by  suture,  but  in  turtles  and 
pterodactyls  the  two  are  fused.  Frequently  vacuities  occur  in 
the  jaw,  and  usually  the  component  bones  can  be  distinguished. 
In  a  few  dinosaurs  a  predentary  or  mento-meckelian  bone  occurs 
at  the  symphysis  of  the  lower  jaw  (Fig.  310). 


FlG.  296.  Sternum  and  shoulder  girdle  of  lizard,  Iguana,  from  Huxley,  cl, 
clavicle;  cr,  coracoid  ;  ecr,  epicoracoid;  gl,  glenoid  fossa;  id,  episternum ;  mcr> 
mesocoracoid ;  msc,  mesoscapula;  sc,  scapula;  St,  sternum;  xst,  xiphisternum. 

The  hyoid  and  branchial  arches  are  variously  developed,  but 
at  no  time  have  they  gill-supporting  functions.  Frequently  the 
first,  or  first  and  second,  branchial  arches  are  well  developed, 
giving  rise  to  long  cornua  attached  to  the  well-developed 
copula. 

The  pectoral  girdle  is  developed  in  all  reptiles  —  even  the 
limbless  lizards  —  with  the  exception  of  the  ophidia.  Scapula, 
coracoid,  and  precoracoid  are  almost  always  present,  the  latter 
lacking  in  ichthyosaurs,  plesiosaurs,  and  dinosaurs,  while  in 

1  The  fixed  or  free  condition  of  the  quadrate  has  been  employed  in  dividing  the  reptilia 
into  monimostylica  and  streptostylica. 


REPTILES, 


301 


FlG.  297.  Pelvis  of  Hatteria,  after 
Wiedersheim.  /o,  obturator  foramen  ; 
i7,  ilium  ;  ts,  ischium ;  /,  pubis ;  //', 
prepubic  process. 


others,  except  theromorphs,  they  are  represented  by  processes 
upon  the  coracoids.     The    scapula,  except  in   chelonia,   is  ex- 
panded dorsally,  while   the  coracoids   are  flattened,  and  either 
meet   in  the   middle   line  as  in 
the  ichthyosaurs  and  plesiosaurs, 
or  they  may  connect  with  the 
sternum.     A  clavicle  is^usually 
present ;   in   the  turtles  it  may 
enter  into  the  composition  of  the 
plastron   (Fig.    305).      An  epi- 
coracoid  occurs  in  some  lizards 
and  turtles. 

A  pelvis  is  more  constant  in 
appearance  than  is  the  shoulder 
girdle,  vestiges  of  it  (ischia)  appearing  in  certain  snakes.  It  is 
characterized  by  the  great  development  of  the  ilium  and  by 
marked  variations  in  the  pubic  bone,  which  in  all  except  croco- 
diles and  pterodactyls  participates  in  the  formation  of  the  ace- 
tabulum,  In  many  dinosaurs  the  pubis  is  differentiated  into 
pre-  and  postpubic  portions  (Fig.  298)  ;  and  traces  of  the  pre- 
pubis  may  be  recognized  in  many  other  groups,  and  also  in 
birds,  as  anteriorly  directed  processes  arising  from  the  pubis. 

The  fore  and  hind  limbs  are 
much  alike  in  their  general  struc- 
ture, and  distinctively  reptilian 
features  are  most  marked  in  the 
distal  portions.  In  the  lower  rep- 
tiles, as  in  chelonians,  the  carpal 
bones  are  much  as  in  amphibia ; 
but  elsewhere  there  is  a  tendency 
to  fusion,  intermedium  and  cen- 
trales  uniting  with  the  radiale, 
while  the  carpales  are  similarly 
reduced  in  number  by  fusion.  In 
the  hind  limbs  much  the  same 
features  can  be  seen,  except  that  the  tarsal  bones  can  fuse  to 
an  even  greater  extent.  In  both  carpus  and  tarsus  there  is  a 
tendency  for  the  proximal  row  to  become  closely  united  to  the 


FIG.  298.  Pelvis  of  Jguanodon, 
after  Dollo.  a,  acetabulum ;  ?7, 
ilium;  is,  ischium;  po,  postpubis; 
pr,  prepubis. 


302 


CLASSIFICATION  OF   VERTEBRATES. 


radius  and  ulna  or  tibia  and  fibula,  while  the  carpales  and  tar- 
sales  in  the  same  way  become  associated  with  the  metacarpals 
or  metatarsals,  thus  producing  an  intracarpal  or  intratarsal  joint. 
The  modifications  of  metacarpals,  metatarsals,  and  phalanges  are 
more  varied  ;  and  we  may  have  walking-feet,  as  in  most  reptiles, 
swimming-feet  or  paddles,  as  in  ichthyosaurs, 
plesiosaurs,  pythonomorphs,  and  some  turtles, 
or,  as  in  the  pterodactyls,  the  anterior  pair 
may  be  modified  into  supports  for  the  organs 
of  flight.  In  the  swimming-feet  there  is  fre- 
quently a  reduction  in  length  of  the  proximal 
bones,  while  the  number  of  phalanges  may  be 
indefinitely  increased. 

The  heart  is  farther  removed  from  the 
head  than  in  the  ichthyopsida,  and  the  sinus 
venosus  becomes  connected  with  the  right  au- 
ricle. Into  the  sinus  empty  the  post-  and  the 
two  precavae,  except  in  the  ophidia,  where 
the  left  precava  opens  directly  into  the  au- 
ricle. The  greatest  advance  is  seen  in  the 
development  of  a  partial  or  complete  (croco- 
diles) septum,  dividing  the  ventricle  into  right 
and  left  halves.  Even  when  the  septum  is 
incomplete  the  ventricle  is  actually  divided 
in  contraction,  the  right  side  containing  only 
venous  blood,  while  the  left  receives  that  re- 
turning from  the  lungs.  Associated  with  the 
division  of  the  ventricle  is  a  corresponding 
division  of  the  ventral  aorta  of  the  ichthy- 
opsida into  three  trunks,  two  connected  with 
the  right  and  one  with  the  left  ventricle.  One 
of  those  arising  from  the  right  ventricle  forms  the  pulmonary 
artery,  blood  passing  through  it,  by  means  of  the  last  aortic 
arches,  to  the  lungs.  The  other  right  ventricular  trunk  con- 
nects by  means  of  the  fourth  arch  of  the  left  side  with  the  left 
aortic  root.  Thus,  as  will  be  seen,  venous  blood  is  forced  from 
the  right  ventricle  of  the  heart  into  the  lungs  and  into  the 
dorsal  aorta.  The  aortic  trunk  arising  from  the  left  ventricle 


FIG.  299.  Arte- 
rial trunks  of  turtle 
(Emys^y  after  Wied- 
ersheim.  a,  left  aor- 
tic arch;  b>  bronch- 
us ;  /,  to  fore  limbs ; 
h,  to  hind  limbs  ;  r, 
renals;  s,  to  stomach. 


REPTILES.  303 

connects  by  means  of  the  fourth  aortic  arch  of  the  right  side 
with  the  right  radix  aortae,  and  also,  by  both  of  the  third  arches, 
with  the  carotids.  This  insures  the  supply  of  arterial  blood  to 
the  brain,  while  a  part  of  the  same  is  carried  to  the  dorsal  aorta, 
which  consequently  contains  both  venous  and  arterial  blood. 
In  Lacerta  and  a  few  other  forms  the  third  arch  of  either  side 
remains  in  connection  with  the  radices  aortae,  but  in  all  other 
groups  this  connection  is  lost. 

Among  other  peculiarities  of  the  circulation  are  the  per- 
sistence of  a  ductus  Botalli  (p.  187)  in  some  chelonians  and 
crocodilia,  and  the  varying  position  of  the  origin  of  the  sub- 
clavians,  which  may  arise  either  from  the  third  (carotid  arch) 
of  either  side,  or  from  the  right  radix  aortae.  Subclavians  are 
lacking  in  the  ophidia.  A  renal  portal  system  occurs  in  all 
except  the  chelonia,  and  in  chelonia  there  are  two  hypogastric 
veins  ;  in  lacertilia  and  ophidia  but  one.  In  the  latter  group 
the  hypogastric  breaks  up  into  a  plexus  connected  with  the  'fat 
body  ; '  passing  thence  to  the  portal  vein. 

The  permanent  kidneys  of  the  adult  reptile  are  the  meta- 
nephridia  ;  they  are  usually  small,  compact,  or  tabulated,  but  in 
snakes  the  lobulation  may  be  carried  so  far  that  the  lobes  are 
connected  only  by  the  ureter.  In  lizards  the  metanephridia  of 
the  two  sides  are  sometimes  united  behind.  The  mesonephros; 
and  the  Wolffian  duct  are  more  or  less  degenerate,  never  func- 
tioning in  the  adult.  Their  remains  are  more  evident  in  the 
female  than  in  the  male,  the  mesonephros  forming  the  so-called 
'golden  yellow  body.'  A  urinary  (allantoic)  bladder  is  con- 
nected with  the  cloaca  in  turtles  and  lizards,  but  in  other  rep- 
tiles it  is  lacking. 

The  gonads  vary  in  shape  with  the  shape  of  the  body,  being 
broad  in  the  chelonia,  long  in  others.  In  many  forms,  and  this 
is  especially  true  of  the  ophidia,  the  right  gonad  is  larger  and 
in  advance  of  the  left.  The  ovaries  are  penetrated  with  a  vas- 
cular network  of  connective  tissue.  The  oviducts  are  long, 
folded  or  contorted,  and  have  smooth  margined  ostia.  The  ducts 
themselves  are  muscular  and  glandular,  the  glandular  portion 
secreting  the  shell. 

Accessory  reproductive  organs  of  two  types  occur.      In  liz- 


304  CLASSIFICATION  OF   VERTEBRATES. 

.ards  and  snakes  there  are  a  pair*  of  eversible  sacs  (hemipenes) 
opening  into  the  cloaca,  and  when  in  repose  retracted  under 
the  skin  of  the  tail.  In  chelonians  and  crocodiles  there  is  but 
a  single  penis,  formed  by  a  thickened  portion  of  the  ventral  wall 
•of  the  cloaca,  which  is  composed  of  erectile  tissue,  and  can  be 
protruded  from  the  vent.  Both  types  are  grooved  for  transmis- 
sion of  the  seminal  fluid.  The  hemipenes  of  embryo  snakes 
have  often  been  described  as  rudimentary  hind  limbs.  Hatteria 
lacks  a  penis. 

The  eggs  of  reptiles  are  large  and  undergo  a  partial  (mero- 
blastic)  segmentation  ;  the  subsequent  phases  of  development 
being  much  like  that  of  birds.  Most  reptiles  are  oviparous, 
the  eggs  being  deposited  in  sand  or  soil,  and  left  to  hatch  by 
the  heat  of  the  sun.  Some  lizards  and  many  snakes,  however, 
are  viviparous. 

The  following  classification  of  the  Reptilia  follows  most 
closely  that  of  Lyddeker.  The  late  Professor  Cope  recognized 
several  more  orders,  which  seem  to  be  but  sub-divisions  of  the 
theromorpha. 

ORDER   I.     THEROMORPHA. 

Extinct  reptiles,  with  amphicoelous  vertebrae,  the  notochord 
frequently  persisting  intervertebrally ;  with  a  sacrum  composed 
of  from  two  to  six  vertebrae  ;  ribs  bicipital,  their  articulation 
with  the  vertebrae  as  in  mammals  ;  quadrate  immovable  ;  teeth 
in  alveoli,  and  showing  much  differentiation  (occasionally  teeth 
are  lacking)  ;  no  sternum  ;  girdles  solid,  the  pubic  and  ischiatic 
bones  fused  into  a  continuous  os  innominatum  ;  humerus  with 
a  foramen  (entepicondylar)  above  the  inner  condyle. 

The  theriomorphs  were  mostly  terrestrial  vertebrates,  and  are  especially 
interesting,  since  they  show  features  which  make  many  regard  them  as 
having  been  the  ancestors  of  the  mammalia.  The  order  appears  in  the 
Permian,  and  dies  out  in  the  triassic. 

SUB-ORDER  i.     PAREIASAURIA  (COTYLOSAURIA). 

Teeth  homodont,  numerous,  without  diastema;  no  temporal  fossa; 
one  occipital  condyle ;  vertebra  with  remains  of  notochord;  two  sacral 
vertebrae.  Pareiasaiirus,  South  African  Permian ;  Empedias,  Permian  of 


REPTILES. 


305 


Texas  ;  Elginia,  triassic  of  Scotland.  Isodectes,  from  the  coal  of  Ohio,  is 
the  oldest  known  reptile.  This  sub-order  is  regarded  by  Cope  as  ancestral 
to  all  other  reptiles. 


FlG.  300.     Pareiasaurus  baini,  after  Seeley. 

SUB-ORDER  2.     ANOMODONTIA. 

Large  lizard-like,  five-toed  reptiles ;  toothless,  or  with  a  single  pair  of 
canine-like  teeth  ;  no  intercentra ;  five  or  six  sacral  vertebras  ;  a  single 
occipital  condyle ;  supratemporal  fossa  present.  Dicynodon  and  Ouden- 
odon,  South  African  Permo-trias. 


FIG.  301.  Skull  of  Dicynodon,  after  Seeley.  bo,  basioccipital ;  c,  columella; 
f,  frontal ;  in,  infranasal ;  j,  jugal ;  /,  lachrymaj  ;  mx,  maxilla  ;  n,  nasal ;  o,  orbit ; 
/,  parietal;  pf,  postfrontal ;  //,  palatine;  pm,  premaxilla;  pr>  prefrontal ;  ptt 
pterygoid  ;  q,  quadrate  ;  s,  squamosal ;  /,  temporal  fossa ;  v,  vomer. 


306  CLASSIFICATION  OF   VERTEBRATES. 


SUB-ORDER  3.     PLACODONTIA. 

Palatine  teeth  large,  pavement-like ;  premaxilla  with  incisors,  maxilla 
with  rounded  molars  ;  lower  jaw  with  incisors  and  pavement  teeth  ;  one  oc- 
cipital condyle.  The  rest  of  the  skeleton  is  unknown.  Placodus,  European 
trias. 

SUB-ORDER  4.     THERIODONTIA  (PELYCOSAURIA). 

Teeth  differentiated  into  incisors,  canines,  and  molars  ;  intercentra  fre- 
quently present ;  supra-  and  infratemporal  fossae  developed  ;  two  or  three 
sacral  vertebrae  ;  carnivorous.  Clepsydrops,  Permian  of  Texas  and  Illinois; 
Dimetrodon  and  Naosaurus  from  the  Permian  of  Texas,  both  with  enormous 
spinous  processes  ;  in  the  latter  these  bear  several  transverse  bars  ;  Gale- 
saurus,  trias  of  Africa. 

ORDER   II.     PLESIOSAURIA   (SAUROPTERYGII). 

Extinct  aquatic  reptiles,  apparently  with  naked  skin  ;  the 
tail  short,  the  neck  long ;  a  single  occipital  condyle ;  temporal 
fossa  present ;  teeth  in  alveoli,  quadrate  immovable ;  anterior 
nares  separate,  near  orbit ;  a  parasphenoid  sometimes  pres- 
ent ;  no  sclerotic  ring  in  orbit ;  vertebrae  amphiccelous  or  flat ; 
ribs  with  a  single  head ;  abdominal  ribs  present ;  sternum  and 


FIG.  302.     Restoration  of  Plesiosaurus,  after  Dames. 

precoracoid  absent,  the  coracoids  meeting  in  the  middle  line ; 
feet  pentadactyl  and  usually  modified  into  swimming-organs. 
The  plesiosaurs  were  large  carnivorous  reptiles,  sometimes 
reaching  a  length  of  forty  feet.  In  Nothosaurus  and  Lario- 
saurus  the  feet  were  fitted  for  creeping,  and  the  animal  was 
lizard-like  ;  triassic  of  Europe.  In  Plesiosaurus  the  limbs  were 
flipper-like,  the  phalanges  being  greatly  increased  in  number, 
while  the  neck  was  extremely  long.  Allied  genera  are  Cimolio- 


REPTILES. 


307 


saurus  and  Pliosaurus   from    the  Jurassic    and    cretaceous    of 
Europe,  America,  and  New  Zealand. 

ORDER  III.     CHELONIA    (TESTUDINATA). 

Recent  and  fossil  reptilia  in  which  the  trunk  is  enclosed  in 
a  bony  framework,  composed  of  a  dorsal  carapace  and  a  ven- 
tral plastron,  these  parts  of  dermal  and  partly  of  endoskeletal 
origin  ;  the  quadrate  is  fixed  ;  teeth  are  lacking,  the  jaws  being 
covered  with  a  horny  sheath.  The  anterior  bony  nares  are 


FIG.  303.  Pectoral 
girdle  of  Plesiosaurus, 
after  Zittel.  c,  coracoid; 
cl,  clavicle ;  *?,  episternum ; 
s,  scapula. 


FIG.  304.  Dorsal  view  of  carapace  of 
green  turtle,  Chelone  midas,  showing  the  ribs, 
RI  extending  beyond  the  costal  plates,  C.  M, 
marginal  plates;  Ntty  nuchal;  Py,  pygal 
plates.  From  Huxley. 


united,  and  open  at  the  tip  of  the  snout.  A  temporal  fossa  is 
frequently  present.  The  scapular  and  pelvic  arches  are  internal 
to  the  ribs.  The  feet  have  five  digits  and,  while  usually  fitted 
for  walking  and  provided  with  claws,  are  occasionally  modified 
into  flippers. 

The  turtles  and  tortoises  are  strongly  marked  off  from  all 
other  reptiles,  the  armor  surrounding  the  body  being  especially 
characteristic.  In  most  cases  head,  legs,  and  tail  can  be  re- 
tracted into  this,  and  in  the  box  tortoises  the  plastron  is  hinged 


308  CLASSIFICATION  OF   VERTEBRATES. 

so  that  it  can  still  further  protect  these  parts.  In  the  atheca 
the  body  is  covered  by  a  thick  leathery  skin ;  but  in  the  others 
it  bears  bony  epidermal  scales  or  plates,  the  arrangement  of 
which  is  of  systematic  importance.  These  plates  in  one  species 
(Eretmoclielys  imbricata)  furnish  the  well-known  '  tortoise 
shell.'  There  is  a  median  and  a  pair  of  lateral  rows  of  plates 
on  the  dorsal  surface,  while  around  the  edge  is  a  series  of  margi- 
nal plates.  Beneath  these  scales  comes  the  dermal  skeleton  of 
bony  plates  ,which,  however,  do  not  correspond  in  position  to  the 
epidermal  coating.  In  the  atheca  this  dermal  skeleton  is  free 
from  the  ribs  and  vertebrae,  and  consists  of  longitudinal  rows 

of  polygonal  dermal  ossicles.  In  all 
others  the  dorsal  portion  of  the  armor, 
the  carapace,  consists  of  a  median  row 
of  (usually  eight)  neural  plates,  each 
being  the  expanded  end  of  a  neural 
spine  of  a  vertebra.  In  front  of  the 
first  neural  is  a  nuchal  plate,  while  be- 
hind the  last  are  two  or  three  pygal 

FIG.  305.  "  Plastron  of      plates,    these    being    unconnected   with 
Chelone  midas,  after  Zittel.      the  vertebrae.      On  either  side  and  cor- 

<    clavicle;  ,,  episternum;  ndi        to  the  neurals  are  the  CQstal 

///,  hypoplastron ;  hy,  hyo- 

plastron;  .r,  xiphopiastron.      plates,  each  fused  to  a  rib;  around  the 
margin    of    the    carapax  is   a   series   of 

marginal  plates,  the  nuchal  and  posterior  pygal  forming  parts 
of  the  series.  The  ventral  portion  of  the  armor,  the  plastron, 
usually  consists  of  nine  plates,  —  in  front  a  median  episternum 
(entoplastron)  flanked  on  either  side  by  a  clavicle  (epiplastron), 
while  behind,  on  either  side,  follow  hyoplastron,  hypoplastron, 
and  xiphiplastron.  Occasionally  the  episterum  is  lacking.  All 
of  these  plates,  except  neurals  and  costals,  are  membrane  bones. 
Besides  the  characters  quoted  in  the  diagnosis,  the  absence 
of  ali-,  pre-,  or  orbitospheneid  ossifications  ;  the  distinct  pro-  and 
opisthotic  bones  ;  and  the  absence  of  an  os  transversum,  —  are 
distinctive.  The  epiotic  is  fused  to  the  supraoccipital ;  a  tem- 
poral fossa  is  usually  present,  but  as  in  chelydosauria,  it 
may  be  absent,  or  again,  as  in  Chelone,  it  may  be  arched  over 
by  an  expansion  of  the  parietal  reaching  to  the  squamosal. 


REPTILES.  309 

The  vertebrae  are  mostly  procoelous,  but   some  of  them  may 
have  plain  faces.     There  are  two  sacral  vertebrae. 

The  position  of  the  girdles  inside  the  ribs  is  secondary,  and 
is  produced  during  growth  by  the  forward  and  backward  ex- 
tension of  the  carapace.  The  ribs  have  but  a  single  head,  and 
extend  into  the  caudal  region  (Fig.  155^).  The  procoracoid  is 
fused  to  the  scapula,  the  carpus  is  primitive,  but  the  tarsus  is 
modified  by  fusion  of  its  ossicles.  Five  digits  always  occur,  but 
the  number  of  phalanges  is  not  constant. 

The  brain  has  large  hemispheres  which  cover  the  twixt  and 
partly  the  mid  brain  ;  the  cerebellum  is  a  slightly  arcuate  trans- 
verse fold.      The   facialis 
and  acusticus  nerves    are  „ 
united     at     their     origin. 
Both  Harderian  and  lach- 
rymal   glands    occur,   the 
latter  being  at  the  poste- 
rior   angle    of    the    orbit.  FIG.  306.     Skull  of  turtle,  Chrysemys  picta. 

The  tympanum  is  well  de-      ^'basiocciPital;  /  frontal;  ".  exoccipital;  /, 

jugal;     /«,    maxillary;     «/,    naso-prefrontal ; 

veloped,  the  membrane  is        /,  parietal;  //,  postfrontal;  //,  pterygoid;  q, 
visible  externally,  and  the        quadrate;  s,  squamosal;  so,  supraoccipital. 

Eustachian  tube  is  large. 

The  ventricular  septum  is  poorly  developed ;  the  third 
aortic  arches  are  not  connected  with  the  radices  aortae,  and  the 
left  radix  gives  off  the  cceliac  artery  before  joining  with  its 
fellow.  A  renal  portal  system  is  lacking,  the  caudal  vein 
connecting  with  the  epigastrics.  The  sexual  and  urinary  ducts 
empty  into  the  neck  of  the  urinary  bladder.  The  penis  is  an 
unpaired  structure  arising  from  the  dorsal  wall  of  the  cloaca, 
and  in  it  are  two  canalicular  extensions  of  the  coelom,  which 
open  on  two  papillae  to  the  exterior. 

The  eggs  are  covered  with  a  leathery  calcareous  shell,  and 
are  buried  in  the  sand,  being  hatched  by  the  heat  of  the  sun. 

Some  of  the  chelonia  are  herbivorous,  some  feed  on  insects, 
molluscs,  etc.,  and  some  are  strictly  carnivorous.  All  are  rather 
slow  in  their  motions  ;  and  the  group  is  best  represented  in  the 
tropics,  the  colder  temperate  regions  having  but  few  species. 
In  cold  climates  the  species  undergo  a  hibernation,  and  in  the 


310  CLASSIFICATION  OF   VERTEBRATES. 

tropics  the  terrestrial  species  sleep  through  the  dry  season 
The  group  appears  in  the  Permian  of  North  America,  and  has 
continued  until  the  present. 

% 

SUB-ORDER  i.     CHELYDOSAURIA. 

No  temporal  fossa ;  carapace  of  transverse  osseous  arches  in  close  con- 
tact, extending  across  the  back  from  side  to  side.  Vertebrae  amphicoalous ; 
limbs  ambulatory.  This  sub-order,  represented  by  Otoccelns  from  the  Per- 
mian of  North  America,  is  regarded  by  Cope  as  ancestral  to  the  other 
chelonia  and  the  pseudosuchian  crocodilia. 

SUB-ORDER  2.     ATHEC^E. 

Turtles  without  scales  but  with  a  leathery  skin,  carapace  of  polygonal 
dermal  bony  plates  arranged  in  rows,  unconnected  with  ribs  and  vertebrae ; 
plastron  poorly  developed,  with  large  central  fontanelle  ;  episternum  lacking. 
Skull  without  descending  process  df  parietals.  Feet  flipper-like,  claws  lack- 
ing. Dermochelys  (Sphargis)  coriacea,  the  leather-back  tortoise,  occurs 
in  all  warmer  seas,  extending  north  to  Cape  Cod.  It  weighs  occasionally 
1,500  Ibs.  The  sub-order  appears  (Psephoderma)  in  the  trias.  Protostega, 
cretaceous  of  Kansas. 

SUB-ORDER  3.     TRIONYCHIA. 

Turtles  with  the  carapace  poorly  ossified,  ribs  and  vertebrae  being 
connected  with  it.  Scales  lacking,  the  body  covered  with  a  leathery  skin ; 
marginal  bones  few  or  absent.  Plastron  with  episternum  and  a  large  me- 
dian fontanelle ;  sacral  and  caudal  ribs  articulating  with  neural  arches. 
A  descending  process  of  the  parietals  present.  Feet  webbed,  three  claws 
on  each  foot.  The  sub-order  appears  in  the  upper  cretaceous  of  New 
Jersey,  and  is  represented  by  over  thirty  species  to-day,  all  inhabitants  of 
fresh  water,  and  best  developed  in  the  Oriental  regions.  All  are  carnivor- 
ous. Four  species  of  leather  turtle  (A  my  da)  and  soft-shell  turtles  (As- 
pidonectes}  in  the  U.  S. 

SUB-ORDER  4.     CRYPTODIRA. 

Turtles  with  well-ossified  carapace,  connected  with  internal  skeleton ; 
epidermal  scales  and  marginal  ossicles  present ;  an  episternum ;  pelvis  free 
from  plastron ;  caudal  ribs  articulated  to  vertebral  centra.  A  descending 
process  to  the  parietals.  The  species  are  numerous,  the  more  important 
families  being  the  following:  CHELONID^E,  with  heart-shaped  carapace,  and 
paddle-like  feet,  bearing  at  most  two  claws.  The  costal  plates  do  not 
reach  the  marginals.  Thalassochelys  caretta,  the  loggerhead  turtle,  weighs 
450  Ibs.  Eretmochelys  imbricata,  the  tortoise-shell  turtle,  is  smaller.  The 
green  turtle,  Chelone  my  das,  may  weigh  850  Ibs.  It  is  highly  esteemed  as 


REPTILES.  3  I  I 

food.  All  of  these  occur  in  the  warmer  Atlantic,  the  tortoise-shell  ranging 
to  the  Indian  Ocean,  and  all  occasionally  occur  on  our  shores.  TESTUDI- 
NID;£,  carapace  strongly  arched;  plastron  very  broad;  five  toes  in  front, 
four  behind.  Terrestrial,  represented  in  southern  U.  S.  by  the  gopher 
turtle,  Xerobates polyphemus.  Here  also  belong  the  giant  tortoises  (Tes- 
tudo  elephantopus,  etc.)  of  the  Galapagos  Islands  and  Mozambique,  and 
the  colossal  fossil,  Colossochelys  atlas,  of  the  upper  miocene  of  India, 
which  was  18-20  feet  long,  the  carapace  being  8  feet  high.  KINOSTER- 


FiG.  307.     Snapping-turtle,  Chelydra  serpentina,  from  Huxley. 


with  free  toes,  short  tail;  9  or  1 1  plates  on  plastron,  with  our  mud- 
turtle  (Kinosternon  pennsylvanicuiri)  and  our  musk-turtles  (Aromochelys}. 
EMYD.E  with  12  plates  on  plastron,  including  about  80  species,  among 
them  the  wood  and  spotted  tortoises  (Chelopus],  the  painted  turtle  (Chrys- 
ernys},  the  box-turtles  (Cistudo)  with  hinge  in  plastron,  and  the  various 
terrapins,  including  the  famous  'diamond  back'  (Malacleinmys pahistris}. 
CHELYDRID^E  with  long  tail,  and  9  plates  on  the  plastron.  Embraces  three 
species,  two,  the  snapping-turtle  (Chelydra  serpentind}  and  the  alligator 
snapper  (Macrochelys  lacertina]  being  the  fiercest  of  reptiles  inhabiting  the 
U.  S.  The  Cryptodira  are  found  in  all  ages  from  the  Jurassic  down. 

SUB-ORDER  5.     PLEURODIRA. 

Turtles  with  epidermal  scales  ;  carapace  united  to  skeleton  ;  marginals 
present ;  caudal  ribs  articulated  to  centrum  ;  descending  process  of  parie- 
tals  present;  neck  bending  horizontally;  pelvis  anchylosed  to  carapace 
and  plastron;  plastron  always  with  13  epidermal  plates.1  Contains  over  50 
species  confined  to  the  southern  hemisphere,  mostly  South  American,  among 
them  Podocnemis,  Chelys,  Pelomedusa.  Sternothcerus  is  African.  While 
the  living  forms  are  very  distinct,  the  fossils  show  intergradations  between 
the  Cryptodira  and  Pleurodira.  Proganochelys,  triassic  of  Germany ;  Both- 
remys,  upper  cretaceous  of  New  Jersey. 

1  Except  in  Carettochelydae,  in  which  epidermal  plates  are  lacking. 


312  CLASSIFICATION  OF   VERTEBRATES. 

ORDER   IV.     ICHTHYOSAURIA   (ICHTHYOPTERYGIA). 

Extinct  aquatic  reptiles  with  naked  skin,  large  head,  short 
neck,  long  bilobed  tail  and  flipper-like  appendages.  Amphi- 
coelous  vertebrae ;  no  sacrum  ;  vertebral  column  extending  into 
lower  lobe  of  tail ;  no  sternum  ;  ribs  bicipital,  abdominal  ribs 
present;  quadrate  immovable ;  jaws  long  and  pointed,  the  upper 
jaw  composed  chiefly  of  premaxillae  ;  teeth  usually  numerous 
(absent  in  Raptaiwdori)  and  seated  in  a  common  groove. 


FIG.  308.  Skull  of  Ichthyosaurus,  after  Zittel.  a,  angulare;  d,  dentary; 
j,  jugal ;  /,  lachrymal ;  7/uc,  maxillary ;  «,  nostril ;  na,  nasal ;  pa,  parietal ;  pft  post- 
frontal  ;  pnt,  premaxillary ;  po,  postorbital ;  p,  prefrontal ;  qj,  quadratojugal ;  s, 
squamosal ;  sa,  supraangulare  ;  st,  stapes. 

The  neural  arches  of  the  vertebrae  are  united  by  suture  to 
the  centra ;  the  caudal  vertebrae  have  chevron  bones  ;  supra- 
temporal  fossa  and  parietal  foramen  are  present.  The  orbits 
are  very  large,  and  contain  a  ring  of  sclerotic  bones  ;  the  exter- 
nal nostrils  are  just  in  front  of  the  orbits.  The  prefrontals  are 
as  large  as,  or  larger  than,  the  frontals  ;  the  pterygoids  extend 
forward  between  the  palatines  to  the  vomers,  and  a  large  para- 
sphenoid  is  present. 

The  coracoids  meet  in  the  middle  line ;  procoracoids  are 
lacking.  The  pelvis  is  entirely  free  from  the  vertebral  column, 
and  its  elements  are  reduced.  The  limbs  are  very  short  and 
paddle-shaped,  the  radius,  ulna,  tibia,  and  fibula  being  reduced 
to  polygonal  bones,  distinguishable  only  by  position  from  the 
metapodial  elements.  The  digits  are  usually  five,  but  this  num- 
ber is  sometimes  apparently  increased  either  by  fission  or  by 
formation  of  marginal  rows  ;  the  phalanges  are  very  numerous. 


REPTILES.  3  1 3 

Ichthyosaurian  coprolites  (Fig.  41)  show  that  these  animals  pos- 
sessed a  spiral  valve,  while  the  finding  of   embryos  within  the 
fossil  skeleton  shows 
that    at    least     some 
species   were    vivipa- 
rous. 

Ichthyosaurians 
were  widely  distribut- 
ed, fossils  having  been  FlG>  309>  Restoration  of  Ichthyosaurus, 

found  in   all  parts  of  after  Fraas. 

the    globe     except 

South  America.     In  time  they  ranged  from  the  upper  triassic 

to  the  upper  cretaceous.     Some  reached  a  length  of  30  or  40 

feet.      Over    50  species  have  been  described.      Ichthyosaurus, 

Mixosaurus,  and  Baptanodon  (Jurassic   of   Wyoming)  are  the 

best  known. 

ORDER   V.     RHYNCHOCEPHALIA. 

Lizard-like,  scaly  reptiles  with  long  tail ;  amphicoelous  verte- 
brae with  frequent  intercentra  ;  ribs  one-headend,  with  uncinate 
processes ;  abdominal  ribs ;  sternum  and  episternum  present  ; 
two  sacral  vertebrae  ;  quadrate  immovable  ;  supra-  and  infratem- 
poral  fossae  present ;  no  procoracoid  ;  limbs  pentadactyl,  vent 
transverse  ;  heart,  lungs,  and  brain  as  in  lacertilia. 

This  order,  is  represented  to-day  by  but  a  single  living 
species,  Sphcnodon  (^Hatteria)  punctata,  from  the  New  Zealand 
region.  While  in  general  appearance  it  is  lizard-like,  it  differs 
much  from  them  in  structure,  and  finds  its  nearest  relatives  in 
fossil  forms  which  range  from  the  trias  to  the  present  time. 
From  the  fact  that  all  the  remaining  groups  of  reptiles  have 
probably  sprung  from  a  rhynchocephalian  ancestry,  the  order 
becomes  very  important,  despite  its  small  size. 

The  vertebrae  are  usually  amphicoelous,  and  remains  of  the 
notochord  occasionally  persist  intervertebrally.  Sometimes  they 
are  flat,  and  in  Proterosaurus  the  cervicals  are  opisthocoelous. 
Intercentra  occur  in  the  caudal  and  cervical  regions,  and  occa- 
sionally in  the  region  of  the  trunk.  A  proatlas  (p.  1 43)  occurs. 
The  premaxillae  are  never  anchylosed ;  the  jaws  bear  acrodont 


3 14  CLASSIFICATION  OF   VERTEBRATES. 

\ 

teeth  or  are  toothless,  and  occasionally  teeth  occur  on  the  ossi- 
fied palatines  (Fig.  171).  The  feet  are  either  fitted  for  walking 
or  for  swimming. 

SUB-ORDER  i.     SPHENODONTINA. 

Small  terrestrial  forms  with  amphicoelous  vertebrae.  Here  belongs  the 
living  Sphenodon.  The  fossil  forms,  Hom<zosaurus,  Hyperodapedon,  Pro- 
terosaurus,  Paloeohatteria,  Telerpeton,  etc.,  have  been  found  only  in 
Europe. 

SUB-ORDER  2.     CHORISTODERA. 

Aquatic  reptiles  with  flattened  vertebras  ;  teeth  on  the  palatines  and 
pterygoids.  Large  forms  from  the  upper  cretaceous  of  North  America 
(Champ os aurus^)  and  lower  eocene  of  Europe  \Simcgdosaurus). 


ORDER   VI.     DINOSAURIA. 

Extinct,  mostly  terrestrial  reptiles,  frequently  of  enormous 
size,  with  long  tail,  ambulatory  feet,  and  a  skin  either  naked  or 
covered  with  large  dermal  spines,  plates,  and  ossicles.  Vertebrae 
solid  or  hollow  ;  flat,  amphicoelous,  or  opisthocoelous,  the  latter 
predominating ;  sacrum  of  3  to  6  vertebrae  ;  ribs  bicipital ;  ab- 
dominal ribs  occasionally  present ;  quadrate  fixed  ;  supra-  and  in- 
f  ratemporal  fossae  present.  Teeth  in  alveoli  or  alveolar  grooves  ; 
no  episternum  or  procoracoid  ;  sternum  partially  ossified  ;  ilium 
elongate  in  front  of  and  behind  the  acetabulum,  the  acetab- 
ulum  itself  open  ;  pubis  with  frequently  a  well-developed  post- 
pubic  branch  ;  toes  with  claws  or  hoofs. 

As  the  name  implies,  the  dinosauria  were  enormous  reptiles, 
remains  of  which  have  been  found  in  all  continents  except  Aus- 
tralia, but  which  were  especially  developed  in  western  America. 
In  many  respects  they  resemble  the  lizards ;  in  others  some 
were  decidedly  bird-like.  In  size  they  varied  between  forms  a 
yard  in  length  up  to  giants  over  a  hundred  feet  long.  Some 
were  herbivorous,  some  carnivorous,  and  they  were  largely  in- 
habitants of  swampy  places  ;  some,  like  Amphicoelias  and  the 
Megalosaurs,  having  bones  so  hollow  and  light  that  it  seems  as 
if  they  could  only  support  the  weight  of  the  body  when  it  was 
immersed  in  water. 

An  exoskeleton  occurred  only  in  some  orthopoda,  and  pos- 


REPTILES.  3  I  5 

sibly  in  some  theropoda.  It  consisted  of  separate  ossicles  or 
bony  plates,  some  of  these  upon  the  back  of  the  stegosaurs 
measuring  a  yard  across.  These  forms  also  had  large  bony 
spines  on  the  tail  as  weapons  of  offence  and  defence. 

While  as  a  rule  the  vertebrae  were  amphicoelous,  occasionally 
those  of  the  neck  were  opisthocoele,  the  rest  being  flat.  Rarely 
procoelous  vertebrae  occurred  in  the  tail.  As  a  rule,  there  were 
10  cervical,  18  trunk,  3  to  6  sacral,  and  30  to  50  caudal  verte- 
brae. A  double  proatlas  occurred  in  the  neck,  and  occasionally 
the  number  of  sacrals  exceeded  6,  there  being  10  in  Polyonax. 
The  caudals  frequently  bore  chevron  bones,  forked  at  their  free 
ends.  The  ribs  are  without  uncinate  processes,  and  abdominal 
ribs  occur  only  in  theropoda.  Usually,  as  in  the  crocodiles, 
there  is  a  preorbital  fossa  between  the  eyes  and  the  nostrils ; 
teeth  occur  only  on  the  jaws.  Clavicles  and  an  episternum  are 
known  only  in  Ignanodon.  The  scapula  is  large,  the  coracoid 
small  and  discoid.  The  anterior  limbs  are  frequently  much 
shorter  than  the  posterior,  and  these  forms  must  have  had  bi- 
pedal or  kangaroo-like  habits.  The  feet  are  digitigrade  or  plan- 
tigrade ;  the  metapodial  bones  are  variously  modified,  and  the 
feet  are  pentadactyl,  although  in  many  but  three  toes  were 
functional. 

The  brain  cavity  shows,  according  to  Cope,  that  the  brain 
.vas  exceedingly  small,  while  the  neural  canal  in  the  sacral  region 
was  much  larger  than  the  brain  cavity  (ten  times  as  large  in 
some  stegosaurs),  implying  a  great  lumbar  enlargement  of  the 
cord.  At  least  some  of  the  order  (Compsognathus)  were  vivi- 
parous. The  group  is  confined  to  mesozoic  rocks,  and  attained 
its  greatest  development  in  the  upper  cretaceous. 

SUB-ORDER  i.     SAUROPODA. 

Large  Dinosaurs  with  the  fore  feet  little  shorter  than  the  hind  ;  anterior 
vertebrze  opisthocoelous,  the  others  amphicoelous  or  flat,  the  centra  with 
large  lateral  cavities.  A  large  preorbital  fossa,  nares  elongate ;  premaxilla 
toothed,  teeth  in  alveoli,  spatulate,  the  sharp  margins  smooth.  No  post 
pubis ;  bones  of  the  extremities  massive,  femur  without  inner  trochanter, 
all  feet  plantigrade,  pentadactyl,  digits  clawed.  This  sub-order  contained 
the  largest  land  animals  known,  most  or  all  of  them  probably  being  herbiv- 
orous. Amphiccelias  {Brontosaurus},  Camarasaurus  (Atlantosaurus},  and 


3l6  CLASSIFICATION  OF   VERTEBRATES. 

Diplodocus,  from  the  upper  Jurassic  of  Wyoming  and  Colorado,  are  the 
best-known  forms,  one  species  of  Caniarasaurus  measuring  115  feet  in 
length.  Only  fragments  are  known  of  the  European  species. 

SUB-ORDER  2.     THEROPODA. 

Moderate-sized  digitigrade  carnivorous  dinosaurs  with  short  fore  limbs 
and  long  tail;  vertebrae  massive  or  hollow,  the  anterior  ones  opistho-  or 
amphiccele ;  a  large  preorbital  fossa,  nares  large,  lateral ;  premaxilla 
toothed,  teeth  pointed,  dagger  shaped,  with  serrate  margins,  seated  in 
alveoli ;  no  postpubis ;  bones  of  extremities  hollow ;  femur  with  inner 
trochanter,  digits  5  or  3,  with  claws.  The  species  range  in  size  from  that 
of  a  cat  (Compsognathus}  to  that  of  an  elephant  (Megalosaurus}.  In 
Europe  the  sub-order  was  restricted  to  the  triassic  and  Jurassic ;  in  Amer- 
ica from  the  triassic  to  the  upper  cretaceous.  Allosaurtis,  Megalosaurus 
(Lcelaps},  and  Ceratosaurus  are  the  best-known  American  genera.  Prob- 
ably some,  at  least,  of  the  famous  footprints  of  the  triassic  of  the  Con- 
necticut Valley  and  New  Jersey  were  made  by  forms  belonging  to  this 
sub-order. 

SUB-ORDER  3.     ORTHOPODA. 

Large  herbivorous  dinosaurs  with  solid  vertebras ;  preorbital  fossa 
small  or  absent ;  nares  large,  anterior ;  premaxilla  toothless,  or  with  teeth 
behind,  lower  jaw  with  an  anterior  toothless  predental  bone  ;  teeth  flattened,. 
the  edges  serrate.  Fore  legs  usually  very  short,  the  hind  feet  with  three 
(rarely  four)  functional  toes.  The  sub-order  is  divided  into  three  sections  : 

A.  STEGOSAURIA. 
Plantigrade  orthopods  with  a 
well-developed  exoskeleton  ;• 
the  bones  massive,  the  verte- 
bras flat  or  amphicrelian. 
Prepubes  not  united  in  front, 
postpubes  slender  and  par- 
allel to  ischium.  Terminal 
phalanges  hoof-like.  Best 
known  are  Scelidosaurus, 
from  the  lower  lias  of  Eng- 
land, and  Hypsirhoph  us 

(Stegosaurus\,  from  the  Ju- 
f 

£  CERATOplfr  Plan! 
tigrade  orthopods  with  well-developed  dermal  skeleton,  sometimes  forming 
a  complete  cuirass.  Bones  massive ;  vertebrae  flat :  skull  with  pointed  pro- 
cesses on  the  frontals  and  with  the  parietals  broadly  expanded  posteriorly. 
A  '  rostral  bone '  in  front  of  the  premaxilla.  No  postpubis ;  prepubes 
distally  expanded  and  united  by  symphysis.  Femur  without  third  tro- 


REPTILES. 


317 


chanter.  The  ceratopsia  are  best  developed  in  the  upper  cretaceous  of 
the  western  U.  S.,  but  have  also  been  found  in  Austria.  They  were  won- 
derfully protected  by  their  armor  and  the  frontal 
horns  against  enemies.  Agathaumas  (Cera- 
tops]  and  Polyonax  (Triceratops),  from  the 
Laramie  beds,  are  best  known.  C.  ORNI- 
T  HO  POD  A.  Digitigrade  orthopods  without 
exoskeleton.  Vertebrae  of  neck  opisthocoele  ; 
fore  legs  very  short ;  prepubes  free,  postpubes 
slender  and  parallel  to  ischium ;  bones  of  ex- 
tremities hollow ;  digits  with  pointed  claws. 
Iguanodon  from  the  upper  cretaceous  of  Eu- 
rope, and  Hadrosaurus  (Diclonhis}  from  the 
green  sand  of  England  and  America,  are  best 
known.  Laosaurus,  upper  Jurassic  of  Wyo- 
ming. One  species  of  Iguanodon  was  33  feet 
long. 

ORDER  VII.     SQUAMATA  (LEPIDO- 
SAURIA,    PLAGIOTREMATA). 

Scaled  reptiles  with  usually  procce- 
lous  vertebrae  ;  ribs  with  a  single  head, 
no  abdominal  ribs  ;  sacrum,  when  pres- 
ent, consisting  of  two  vertebrae  ;  quad- 
rate free  ;  supratemporal  fossa  present 
or  postf rental  arch  incomplete  ;  jugal 
arch  always  incomplete  ;  teeth  acrodont  *«u™>  after  c°Pe  <see  F[%' 
or  pleurodont  ;  cerebellum  very  small, 

optic  lobes  approximate  ;  ventricles  of  heart  incompletely  sep- 
arated ;  vent  a  transverse  slit ;  two  hemipenes. 

Lizards  and  snakes  are  frequently  regarded  as  constituting 
two  distinct  orders  ;  but  in  spite  of  the  absence  of  feet  and 
some  other  characters,  the  two  groups  (together  with  the  ex- 
tinct pythonomorphs)  have  so  many  points  in  common  that  the 
order  here  recognized  is  justified.  The  body  is  covered  with 
horny  epidermal  scales,  and  frequently  these  are  re-enforced  by 
dermal  ossifications.  In  only  rare  instances  are  the  vertebrae 
amphiccelous.  The  nasal  apertures  in  the  skull  are  separate  ; 
the  lungs  are  simple  sacs  ;  limbs,  when  present,  are  ambulatory 
or  natatory. 


FIG.  311.     Skull  of  Hadro- 


CLASSIFICA  TION  OF   VER  TEBRA  TES. 


fr 


FIG.  312.  Side  and  sectional  veins  of  skull  of  Cyclodus,  from  Huxley.  Ar, 
articulare;  BS,  basisphenoid ;  BO,  basioccipital ;  Co,  columella;  D,  dentary; 
EO,  exoccipital;  Fr,  frontal;  EpO,  epiotic ;  Ju,  jugal ;  MX,  maxillary  ;  Na,  nasal; 
Pa,  parietal;  Pf,  postfrontal ;  Pmx,  premaxillary ;  PI,  palatine;  Pt,  pterygoid  ;  PrO, 
prootic;  OpO,  opisthotic ;  Prf,  pref  rental ;  Qu,  quadrate;  SO,  supraoccipital ;  Sq, 
squamosal ;  Tr,  transversum ;  Vo,  vomer ;  V,  VII,  passages  for  fifth  and  seventh 
nerves. 

SUB-ORDER  i.     LACERTILIA  (SAURII). 

Scaled  or  plated  reptiles  usually  with  two  pairs  of  feet;  vertebrae 
rarely  amphiccelous  ;  premaxilfa  single  or  paired.  Postorbital  arcade  some- 
times entire,  jugal  arch  never  complete.  Ali-  and  orbitosphenoids  not 
ossified  ;  shoulder  girdle  always  present.  Sternum  and  episternum  usually 
present.  Feet  sometimes  rudimentary  or  absent;  when  present,  usually 
five-toed  and  ambulatory ;  the  -friaxilla,  palatines,  and  pterygoids  cannot 
move  on  the  bones  of  the  skull,  and  the  mouth  can  be  opened  to  but  a 
moderate  extent.  Movable  eyelids,  tympanic  cavity  and  membrane  usu- 
ally occur.  The  arteries  supplying  the  alimentary  canal  are  extremely- 
variable. 

The  lizards  in  their  outward  appearance  resemble  closely  the  crocodiles 
and  Sphenodon,  but  in  structure  they  have  many  and  important  points  of 
difference.  The  apodal  forms  are  strikingly  snake-like ;  but  these  may  be 
distinguished  in  most  cases  at  a  glance  by  the  presence  of  eyelids  and 
small  scales  instead  of  broad  abdominal  scutes  on  the  ventral  surface  of 
the  body.  The  lizards  are  largely  insectivorous,  and  only  one  has  the  repu- 


REPTILES.  319 

tation  of  being  poison.  Most  of  them  lay  eggs  enclosed  in  a  leathery 
shell.  Most  of  the  1,200  living  species  are  confined  to  the  warmer  re- 
gions of  the  earth.  The  sub-order  appears  in  cretaceous,  but  the  fossil 
forms  are  few.  A  natural  classification  of  the  sub-order  is  still  a  desid- 
eratum. That  adopted  here,  based  primarily  upon  the  tongue,  associates 
together  widely  diverse  forms. 

SECTION  I.  VERMILINGUIA.  Old-world  lizards  with  vermiform, 
highly  extensile  tongue ;  tongue  papillose,  its  enlarged  tip  sheathed ;  body 
covered  with  small  chagreen  scales ;  tail  coiling  vertically,  and  used  as 
organ  of  prehension.  No  anal  or  femoral  pores.  Orbits  closed  behind 
by  process  of  jugal ;  teeth  acrodont ;  no  teeth  on  palatines.  Feet  with  the 


FIG.  313.     Head  of  Chamelon  with  the  tongue  extended. 

toes  in  two  groups.  Only  genus,  Chameleon,  with  about  30  species.  Trie 
chameleons  are  noted  for  their  color-changes,  a  feature  which  is  shared  to 
a  marked  extent  by  the  American  genus  Anolis  (infra) .  There  are  two 
pigment  layers  in  the  skin,  an  upper  bright  yellow  and  a  deeper  dark  brown 
or  black  layer.  The  pigment  cells  in  these  layers  are  under  control  of  the 
sympathetic  system,  and  according  as  one  or^he  other  becomes  prominent 
the  color  of  the  animal  changes. 

SECTION  II.  CRASSILINGUIA.  Lizards  with  thick,  short,  fleshy 
tongue,  usually  rounded  at  the  tip  (never  strongly  emarginate),  not  protru- 
sible,  papillose  or  smooth ;  tympanic  membrane  usually  free.  ASCALA- 
BOTjE  or  geckoes  have  the  feet  with  adhesive  disks  on  the  underside,  and 
usually  granular  or  spinose  scales.  Teeth  pleurodont ;  no  teeth  on  pal- 
atines or  pterygoids;  a  circular  fold  in  place  of  eyelids.  Vertebrae  amphi- 
coelous.  The  geckoes  receive  their  common  name  from  their  cry.  They 
occur  in  all  the  warmer  regions  of  the  world  except  the  northeastern  part 
of  the  U.  S.  Possibly  the  group  should  be  more  strongly  marked  off  from 
other  forms.  Phyllodactylus  occurs  in  California.  Other  genera  are  Platy- 
dactylus,  Ptychozoon,  and  Ascalabotes ;  200  species  known.  All  are  in- 
sectivorous and  have  great  powers  of  climbing.  IGUANID/E,  lizards 
of  considerable  size,  without  adhesive  feet ;  body  compressed ;  limbs 
long  and  slender ;  often  a  comb  of  spines  on  the  back  ;  pleurodont  teeth  ; 


320 


CLASSIFICATION  OF   VERTEBRATES. 


teeth  usually  on  pterygoids.  Usually  a  large  brightly  colored  sac  beneath 
the  throat  connected  with  the  hyoid.  Over  300  species  known,  all  but  a 
few  from  the  new  world.  Anolis  includes  the  'chameleon'  (A  caroli- 

nensis)  of  our  southern  states.  Sce- 
leporus  contains  the  common  lizard  or 
swift  (S.  undulatus)  of  the  eastern 
states  north  to  Connecticut  and  Mich- 
igan. The  various  species  of  '  horned 
toad '  belong  to  Fhrynosoma.  The 
AGAMID^E  replace  the  Iguanidae  in 
the  eastern  hemisphere.  One  hundred 
and  fifty  species  are  known.  These 
all  have  acrodont  teeth.  Chlaniydo- 
saurus  includes  the  frilled  lizard,  C. 
kingii,  of  Australia,  with  a  broad 
dermal  fold  or  collar  about  the  neck ; 
Draco  volans  of  Java  has  the  ribs 
greatly  elongate,  supporting  a  fold  of 
skin  which  acts  as  a  parachute. 

SECTION  III.  BREVILINGUIA. 
Tongue  short,  thick  at  base,  no 
sheath  ;  tip  smaller  and  more  or  less 
emarginate  ;  only  slightly  protrusible  ; 
pleurodont  dentition;  feet  often  re- 
duced, two  or  none,  the  toes  also  fre- 
quently reduced  in  number;  but  in 
all  cases  pectoral  and  pelvic  girdles 
are  present.  Over  400  species  are 
known,  but  few  of  them  inhabitants 
of  the  U.  S.  The  SCINCID.E  have  a 
more  or  less  snake-like  body,  covered 

with  smooth  bony  scales ;  tongue  two-pointed.  Eumeces,  with  teeth  on 
the  palate,  contains  our  blue-tailed  lizard  (E.  fasciatus} ;  and  our  weak- 
legged  ground  lizard  belongs  to  the  genus  Oligosoma.  Scincus,  with  five 
toes,  contains  the  true  skinks.  In  Seps  the  toes  are  three  in  number. 
Scelotes  has  only  hinder  extremities,  and  in  Anguis  and  Typhline  legs  are 
lacking.  Cyclodus.  The  ZONURID^:  may  be  recognized  by  a  finely  scaled 
groove  along  the  side  of  the  body.  All  except  our  '  glass  snake,'  Ophi- 
saurus  ventralis,  belong  to  the  old  world.  This  species,  which  is  limbless, 
derives  its  common  name  from  the  brittleness  of  its  tail. 

SECTION  IV.  FISSILINGUIA.  Tongue  long,  slender,  protrusible, 
its  tip  deeply  split ;  eyelids  well  developed  ;  tympanic  membrane  visible ; 
legs  well  developed.  VARANID/E,  pleurodont,  tongue  retractile  into  sheath  ; 
Varanus  (Monitor]  contains  about  30  old-world  species.  LACERTID^E, 
pleurodont,  no  tongue  sheath;  usually  femoral  pores;  Lacerta,  Tropido- 


FiG.  314.     Green  lizard,  Anolis > 
from  Lutken. 


REPTILES. 


321 


saurus.     All  the  species  belong  to  the  old  world.     HELODERMID^E,  pleu- 
rodont ;  tongue  with  papillae  at  base  ;  no  femoral  pores.     Heloderma,  with 
two  species,  horridum  and  suspectum,  from  the  Mexican  region,  contains 
the    only    poisonous    lizards.       TEID^E, 
acrodont,   tongue    two-pointed,    covered 
with  imbricate  scales ;  tympanic  mem- 
brane  visible ;    usually    two    transverse 
folds  on  throat.     Limbs  present,  rarely 
rudimentary.     About  70  American  spe- 
cies.    Cnemidopkorus,  with  rounded  tail, 
eyelids  developed,  small  scales  and  large 
ventral   plates,  includes   the  six-striped 
lizard    (C.  sexli^eatus}  of   the    eastern 
U.S.     Tejus  teguixin  of  Central  Ameri- 
ca reaches  a  length  of  6  or  7  feet. 

SECTION  V.  ANNULATA.  Body 
covered  by  quadrangular  scales,  ar- 
ranged in  rings  around  the  body.  Body 
vermiform,  limbless,  or  with  small  fore 
limbs.  Teeth  acrodont  or  pleurodont, 
no  palatine  teeth  ;  tongue  short,  thick, 
non-protrusible ;  eyelids  and  tympanic 
membrane  lacking.  About  50  species, 
half  of  them  belonging  to  Amphisbcena. 
All  the  species  tropical  or  subtropical ; 
they  live  burrowing  in  the  earth,  and 
feed  especially  on  insects  and  worms. 

The  .Lacertilia  are  poorly  represented  as  fossils,  the  group  appearing  in 
the  cretaceous.  Most  of  the  fossils  are  referred  to  existing  families,  but 
the  Dolichosauria  from  the  cretaceous  of  Europe,  differ  from  all  recent 
lizards  in  having  more  than  nine  cervical  vertebrae. 

SUB-ORDER  2.     PYTHONOMORPHA. 

Large,  extinct,  extremely  elongate  reptiles  with  four  flipper-like  extremi- 
ties ;  vertebrae  proccelous,  with  or  without  zygantra  and  zygosphenes ;  usu- 
ally no  sacrum ;  supratemporal  fossa  present,  jugal  arch  incomplete ;  teeth 
large,  conic,  acrodont,  fused  to  maxillae  and  pterygoids  ;  a  parietal  foramen  ; 
both  girdles  present ;  feet  pentadactyl,  without  claws. 

The  Pythonomorpha  occur  in  the  upper  cretaceous  of  America,  Europe, 
and  New  Zealand.  The  vertebrae  number  more  than  a  hundred  ;  the  cer- 
vicals  bear  strong  hypapophyses,  the  caudals  with  chevron  bones.  The 
skull  was  lizard-like,  the  cranial  cavity  being  open  in  front.  The  parietals 
are  fused  in  the  middle  line,  and  were  connected  with  the  alisphenoids  and 
prootic  by  lateral  processes.  The  quadrates  are  large,  and  are  articulated 
to  a  supratemporal.  The  premaxilla  unpaired,  the  rami  of  the  lower  jaw 


FlG.  315.  Skull  of  ffeloderma, 
after  Giinther.  /,  frontal ;/,  jugal ; 
;//,  maxillary  ;  n,  nasal ;  p,  parietal  ; 
//,  postfrontal ;  //',  prefrontal ;  //, 
pterygoid ;  /.r,  premaxillary ;  s, 
squamosal ;  so,  supraoccipital. 


322 


CLASSIFICATION  OF   VERTEBRATES. 


united  by  ligament  at  the  symphysis.  Sternum  and  episternum  but  rarely 
occur,  the  coracoids  (which  bear  a  procoracoid  process)  meeting  in  the 
middle  line.  The  pelvis  was  weak,  and  in  most 
forms  the  ilium  was  without  connection  with  the  ver- 
tebral column.  The  bones  of  the  limbs  were  short. 
In  most  forms  the  skin  was  naked,  or  at  least  lacked 
dermal  ossicles. 

Two  groups  are  recognized,  —  the  PLIOPLATE- 
CARPID.-E,  with  a  sacrum  of  two  fused  vertebrae  and 
an  episternum ;  and  the  MOSASAURID^E,  in  which  a 
sacrum  was  lacking.  Plioplatecarpus  occurs  in  the 
rocks  of  Maestricht.  Mosasaurus  (species  of  which 
occur  in  Holland,  England,  and  the  eastern  U.  S.) 
was  first  found  over  a  hundred  years  ago.  Clidastes 
from  Alabama,  and  Platecarpus  from  Kansas,  are 
pretty  well  known.  Liodon,  from  the  cretaceous  of 
both  continents.  Some  of  the  phythonomorphs  were 
over  40  feet  in  length. 


FIG.  316.  Skull  of 
Liodon,  after  Owen. 
b,  basioccipital ;  /, 
frontal ;  j,  jugal ;  /, 
lachrymal ;  mx,  max- 
illa;  w,  nasal;  /, 
parietal,  with  large 
parietal  foramen  ; 
pm,  premaxilla ;  pf, 
prefrontal ;  pof,  post- 
frontal  ;  </,  quadrate ; 
s,  squamosal  ;  st,  su- 
pratemporal. 


SUB-ORDER  3.     OPHIDIA  (SERPENTES). 


Footless,  elongate,  scaled  reptiles,  with  proccelous 
vertebrae  ;  without  chevron  bones,  sacrum,  or  pectoral 
girdle  ;  no  parietal  foramen  or  temporal  arch  ;  sternum 
and  tympanum  lacking ;  no  movable  eyelids ;  tongue 
bifid,  protrusible ;  teeth  acrodont ;  no  dermal  osseous 
scutes  ;  no  urinary  bladder. 

Snakes  are  to  be  confused  only  with  the  footless 
lizards,  from  which  they  differ,  however,  in  many 
structural  features.  The  body  is  covered  above  by 
imbricate  scales,  while  the  lower  surface  of  the  body 
is  usually  covered  by  broad  plates,  — the  abdominal  scutes.  The  scuta  on 
the  head  are  regularly  arranged,  and  the  characters  they  present  are  of 
value  in  classification  (Fig.  317).  These  scales  are  regularly  shed,  and  as 
regularly  reformed  by  epidermal  cornification. 

The  procoelous  vertebrae,  which  may  exceed  400  in  number,  bear  zygan- 
tra  and  zygosphenes,  and  can  only  be  divided  into  caudal  and  precaudal 
series.  The  anterior  10  to  30  bear  large  hypapophyses.  The  neural 
arches  are  fused  to  the  centra,  and  the  ribs,  which  are  frequently  hollow, 
begin  with  the  third  vertebrae.  Abdominal  ribs  and  all  sternal  structures 
are  lacking. 

The  bones  of  the  skull  are  very  solidly  ossified,  the  brain  capsule  being 
long,  and  closed  in  front.  Many  of  the  bones  (maxilla,  pterygoids,  supra- 
temporal)  are  loosely  articulated  together.  The  parietals  are  laterally  elon- 
gate, and  fused  with  the  prootic,  ali-  and  orbitosphenoid.  The  opisthotics 


REPTILES. 


323 


are  fused  with  the  exoccipital,  and  the  basi-  and  presphenoids  are  united. 
Post-  and  prefrontals  .and  lachrymals  are  present,  and  the  vomers  are 
paired.  There  is  an  os  transversum,  but  jugals  and  quadrate jugals  are 


FIG.  317.  Diagram  of  the  cephalic  plates  in  a  colubrine  snake.  /,  f renal;  ft, 
frontal ;  i,  internasal ;  il,  infralabial ;  n,  nasal ;  /,  parietal ;  pf,  prefrontal ;  poy 
postorbital ;  pro,  preorbital ;  r,  rostral;  st,  supralabial. 

lacking.  The  premaxillary  is  very  small,  and  there  is  no  columella.  The 
quadrate  is  articulated  to  the  supratemporal  (squamosal  auci},  which  in 
most  forms  has  but  a  loose  connection  with  the  other  bones  of  the  skull. 
Teeth  occur  on  the  maxillae  and  pterygoids,  and  usually  on  the  palatines, 


FlG.  318.  Skull  of  garter-snake  {Eutainia  sirtalis~}.  a,  angulare  ;  d,  dentary; 
e,  ethmoid;  /,  frontal;  ///,  maxillary;  «,  nasal;  /,  parietal;  pa,  palatine;  pm, 
premaxillary ;  pf,  prefrontal ;  po,  postfrontal ;  ps,  parasphenoid ;  //,  pterygoid ;  qy 
quadrate  ;  s,  supratemporal ;  /,  transversum  ;  v,  vomer. 

and  they  may  be  present  on  premaxilla.  The  teeth  are  usually  sharp, 
conic,  and  retrorse,  and  frequently  those  of  the  maxilla  may  be  grooved. 
When  the  grooved  teeth  occur  on  the  anterior  end  of  the  maxilla  (Protero- 
glypha,  Solenoglypha)  they  are  connected  with  poison  glands.  The  rami 


324 


CLASSIFICATION  OF   VERTEBRATES. 


of  the  lower  jaw  are  united  by  ligament  at  the  symphysis,  and  are  capable 
of  wide  separation,  which,  together  with  the  loosely  articulated  cranial  bones, 
allows  of  great  increase  in  the  size  of  the  oral  opening. 

In  only  the  peropoda  are  rudiments  of  hind  limbs  visible  as  small 
stumps  having  small  claws  on  either  side  of  the  vent.  The  pelvic  girdle 
occurs  as  a  rudimentary  structure,  unconnected  with  the  vertebral  column, 

in  the  peropoda  and  Typhlops.  In 
the  latter  but  the  rudiments  of  the 
girdle  occur,  in  the  former  the  pelvis 
is  represented  by  a  slender  bone  with 
which  two  diverging  bones  are  articu- 
lated below. 

The  tongue  of  the  snakes  is 
deeply  forked,  and  is  retractile  into 
a  sheath  on  the  floor  of  the  mouth. 
It  can  be  protruded  when  the  mouth 
is  closed,  thanks  to  a  groove  in  the 
edge  of  the  lips.  The  poison  glands 
which  occur  in  certain  snakes  are 
modified  labial  glands.  Sometimes 
they  are  greatly  enlarged,  and  may 
extend  backwards  into  the  throat. 
They  are  so  placed  that  the  action  of 
the  muscles  which  close  the  jaw  will 

force  the  poison  out  through  a  duct  into  a  groove  in  the  modified  teeth 
which  serve  as  poison  fangs.  In  the  proteroglyphs  these  grooves  are  open  ; 
but  in  the  solenoglyphs  the  edges  of  the  groove  meet,  so  that  a  poison 
canal  is  formed  inside  the  tooth.  The  stomach  is  long,  and  the  intestine 
has  few  convolutions.  The  trachea  is  very  long,  and  often  has  respiratory 
chambers  in  its  course.  The  left  lung  is  rudimentary,  the  right  very  long 
with  an  air  reservoir  at  the  end. 

Locomotion  is  effected  by  the  lateral  bending  of  the  vertebral  column 
and  by  the  ribs,  which  can  be  moved  forward  and  back;  and  as  these  are 
attached  to  the  abdominal  scutes  these  latter  can  be  moved,  and  as  they 
catch  every  irregularity  of  the  surface,  the  animal  is  able  to  propel  itself. 
Snakes  are  all  carnivorous,  the  majority  feeding  upon  vertebrates,  some 
killing  their  prey  by  poison,  some  by  crushing  it.  It  is  swallowed  whole. 
Most  snakes  lay  eggs,  which  are  large  and  enclosed  in  a  leathery  shell ;  but 
a  large  number  are  viviparous.  Snakes,  of  which  nearly  2,000  species  have 
been  described,  have  their  metropolis  in  the  tropics.  In  colder  climates 
they  undergo  a  hibernation  during  the  winter.  The  earliest  snakes  appear 
in  the  cretaceous  ;  but  few  fossils  of  the  group  are  known,  and  these  chiefly 
by  vertebrae,  skulls  being  very  rare. 

SECTION  I.  COLUBRIFORMIA.  Snakes  with  the  supratemporal 
overlapping  the  cranium ;  maxillary  bone  horizontal,  not  erectile  ;  teeth  not 


FIG.  319.  Rudimentary  pelvis  and 
hind  limb  (A)  of  Stenostoma  macro- 
lepis  and  (#)  of  Boa,  after  Fiir- 
bringer.  f,  femur ;  ?7,  ilium  ;  ip, 
'  iliopectineum  ;  '  /,  pubis ;  /,  tibia. 


REPTILES. 


325 


grooved  or  channelled,  or  only  the  hinder  teeth  grooved ;  not  poisonous. 
The  PEROPODA,  with  rudimentary  hinder  limbs,  includes  the  Pythons,  giant 
snakes  of  Africa,  and  Boa  and  Eunectes  (the  anaconda),  equally  large  forms, 
from  South  America.  The  AGLYPHODONTA  resemble  the  Peropoda 
in  the  absence  of  grooved  teeth,  but  differ  in  the  absence  of  limbs.  The 
species  are  numerous ;  our  species  mostly  belong  to  the  COLUBRID/E,  in 
which  the  head  is  distinct  from  the  trunk ;  the  teeth  are  numerous  on  max- 
illaries  and  palatines.  Over  700  species  described ;  about  50  in  north- 
eastern U.  S.  Tropidonotus  includes  our  water-snakes  ;  our  garter-snakes 
belong  to  Eutainia.  Bascanion  contains  the  black  snakes ;  Heterodon, 
the  choleric  but  harmless  spreading  vipers.  Rachiodon  of  Africa  has  the 
vertebral  hypapophyses  tipped  with  enamel, 
forming  a  series  of  teeth  which  penetrate 
the  oesophagus,  and  are  of  use  in  cutting 
open  the  eggs  on  which  these  animals  feed. 
The  OPISTHOGLYPHA  have  some  of  the 
posterior  maxillary  teeth  grooved.  Dipsas 
and  its  allies  are  tropical  arboreal  forms, 
with  nocturnal  habits. 

SECTION  II.  PROTEROGLYPHA. 
Snakes  with  large,  permanently  erect, 
grooved  poison  fangs  on  the  anterior  end 
of  the  maxillaries.  A  poison  gland  is  al- 
ways present.  They  live  in  warm  countries, 
and  are  usually  brightly  colored  (warning 
colors).  The  ELAPID^E  contains  the  coral- 
snake  or  bead-snake,  Elaps  fulvus,  of  our 
southern  states ;  Naja  tripudians,  the 
cobra  of  India;  and  N.  haje,  the  asp,  tra- 
ditionally connected  with  Cleopatra.  The 
HYDROPHID,E  embraces  the  sea-snakes  of 
the  Indian  Ocean,  one  species  ranging  to 
Panama.  These  are  pelagic  throughout 
life,  feeding  upon  invertebrates  and  small 
fishes.  They  bring  forth  their  young 
alive. 

SECTION  III.  SOLENOGLYPHA.  Maxilla  vertical,  in  front  armed 
with  large  erectile  poison  fangs  in  which  the  groove  has  been  converted  by 
folding  into  a  tube.  In  the  VIPERID^E  there  is  no  pit  or  groove  between 
nostril  and  eye.  The  species,  about  20  in  number,  all  belong  to  the  old 
world,  two  species  of  vipers  (Vipera)  and  one  adder  (Pelias}  occurring  in 
Europe.  In  the  CROTALID^:,  the  40  species  of  which  occur  in  America 
and  Asia,  there  is  a  deep  pit,  partly  occupying  a  cavity  in  the  maxillary 
bone,  between  nostril  and  orbit.  Crotalus,  in  which  the  tail  ends  in  a 
rattle  formed  by  the  remains  of  exuviated  skin,  contains  the  rattlesnakes,  of 


FlG.  320.  //,  poison  tooth 
of  rattlesnake  ;  C,  the  same  in 
section  (solenoglyphic);  £,  sec- 
tion of  poison  tooth  of  the 
cobra  (proteroglyphic),  show- 
ing the  groove  (^)  of  closure  in 
formation  of  the  poison  canal, 
c ;  o,  openings  of  poison  canal ; 
/,  pulp  cavity ;  after  Boas. 


326 


CLASSIFICATION  OF   VERTEBRATES. 


which  three  species,  C.  korridus,  C.  adamanteus,  and  C.  catenatus  occur 
in  our  eastern  states.  Agkistrodon  contortrix,  the  copperhead,  and  A. 
piscivorus,  the  moccasin,  lack  the  rattle,  as  does  Bothrops  lanceotatus,  the 
fer-de-lance  of  the  Antilles,  possibly  the  most  deadly  snake. 

SECTION  IV.  TORTRICINA.  Colubriform  snakes  with  supratem- 
poral  articulated  with  bones  of  skull ;  mouth  incapable  of  distention ;  a 
horizontal  maxillary  bone,  rudiments  of  pelvis  and  anal  claw.  The  species, 
belonging  to  Tortrix,  Rhinophis,  Uropeltes,  etc.,  are  all  tropical. 

SECTION  V.  OPODERODONTA.  With  articulated  supratemporal, 
rudimentary  pelvis,  head  and  eyes  small;  bones  of  skull  immovable;  mouth 
incapable  of  distention,  teeth  in  only  upper  or  under  jaw,  body  worm-like, 
tail  very  short.  Seventy  species  in  the  tropics,  where  they  live  a  burrowing 
life  like  earthworms.  Typhlops ;  Stenostoma. 


ORDER  VIII.    CROCODILIA  (LORICATA,  CATAPHRACTA). 

Lizard-shaped  reptiles  with  bony,  dermal  scutes  ;  bicipital 
ribs  ;  a  supra-  and  usually  an  infratemporal  fossa ;  teeth  in  alve- 
oli in  the  edges  of  the  jaws  only  ;  quadrate  immovable  ;  sternum 
and  episternum  present  ;  four,  usually  clawed,  ambulatory  limbs  ; 
tail  long,  keeled  ;  lungs  compound  sacs  ;  vent  longitudinal ;  penis 
unpaired;  heart  with  ventricles  completely  separated. 


FlG.  321.  Skull  of  alligator,  an,  angulare  ;  ar,  articulare  ;  bo,  basioccipital ; 
bs,  basisphenoid ;  d,  dentary ;  fr,  frontal ;  j,  jugal ;  /,  lachrymal ;  »ix,  maxillary ; 
pm,  premaxillary ;  //,  pterygoid ;  q,  quadrate;  qj,  quadratojugal ;  tr,  os  transversum. 

The  crocodiles,  alligators,  and  caimans,  and  their  extinct  rel- 
atives, are  sharply  marked  off  from  all  other  reptilian  groups, 


REPTILES.  327 

except  the  theromorphs,  by  structural  characters,  although  in 
external  appearance  they  are  closely  similar  to  the  lizards. 
They  have  the  vertebrae  pro-  or  amphicoelous  or  amphiplatyan, 
the  atlas  peculiar  in  consisting  of  four  parts ;  cervical  ribs  are 
present,  and  the  sternum  is  largely  cartilaginous.  The  skull  has 
usually  a  corroded  appearance,  an  interorbital  septum  occurs, 
the  premaxillae  are  paired,  and  there  is  no  parietal  foramen. 
Ventral  ribs  are  present ;  the  procoracoid  is  a  process  of  the 
coracoid.  The  acetabulum  is  closed.  All  of  the  bones  are 
solid. 

Dermal  plates  are  best  developed  on  the  back,  but  may  occur 
on  the  ventral  surface  as  well.  They  consist  of  dermal  ossifica- 
tions overlaid  with  epidermal  scales.  The  eyes  have  a  vertical 
pupil,  and  both  lids  and  nictitating  membrane  are  present. 
The  nostrils  and  ears  are  provided  with  valves.  The  teeth 
are  confined  to  the  edges  of  the  jaws,  and  are  never  found 
on  palatines  or  pterygoids.  Salivary  and  lachrymal  glands  are 
lacking ;  the  stomach  is  muscular  and  resembles  somewhat 
that  of  birds.  There  is  no  caecum.  A  peculiarity  is  the  ex- 
istence of  peritoneal  canals  connecting  the  coelom  with  the 
exterior. 

The  crocodiles  and  alligators  are  among  the  largest  of  living 
reptiles,  the  giant  tortoises  alone  rivalling  them.  They  are 
aquatic  and  mostly  fluviatile.  They  capture  their  prey  by  lying 
in  wait  for  it,  usually  with  but  the  eyes  and  the  tip  of  the  nose 
above  water.  In  their  motions  they  are  very  quick.  The 
smaller  forms  live  chiefly  upon  fishes,  but  the  larger  prey  on 
mammals  when  the  chance  comes.  The  eggs  are  laid  either  in 
sand  or  in  rough  nests,  and  vary  in  size  from  those  of  a  hen 
to  those  of  a  goose.  The  group  to-day  is  exclusively  tropical, 
and  has  recently  been  greatly  reduced  in  numbers,  owing  to 
the  desire  for  the  skins.  The  crocodiles  appear  in  the  trias 
(Parasuchia  and  Pseudosuchia). 

SUB-ORDER  i.     PSEUDOSUCHIA. 

Extinct  crocodilia  in  which  the  back  is  covered  with  two  rows  of  oblong 
bony  plates.  Vertebrae  unknown  ;  premaxilla  small  and  thin  ;  nostrils  an- 
terior ;  postorbitals  present ;  no  infratemporal  fossa ;  teeth  few ;  hinder 


328  CLASSIFICATION  OF   VERTEBRATES. 

feet  pentadactyl,  fifth  toe  reduced.  dEtosaurus,  the  best-known  genus, 
comes  from  the  triassic  of  Wiirtemberg.  Typothorax,  triassic,  New 
Mexico. 

SUB-ORDER  2.     PARASUCHIA. 

Extinct  crocodilia  with  amphicoelous  or  flat  vertebrae ;  premaxilla  very 
long ;  external  nostrils  separate  and  near  orbits ;  inner  choana  at  anterior 
end  of  palatines.  Palatines  and  pterygoids  not  meeting  in  the  middle  line. 
Supratemporal  fossa  small,  infratemporal  large ;  parietals  and  frontals 
paired  ;  post  orbitals  present ;  coracoid  short,  discoidal ;  clavicle  present ; 
toes  unknown.  The  species  come  from  the  trias  of  Europe,  America,  and 
the  East  Indies.  Belodon  is  found  in  Wiirtemberg,  Pennsylvania,  North 
Carolina,  and  New  Mexico. 

SUB-ORDER  3.     EUSUCHIA. 

Crocodilia  with  amphi-  or  procoelous  vertebrae ;  premaxilla  short ;  ex- 
ternal nares  united  and  at  front  of  snout ;  palatines  and  usually  pterygoids 
touching  in  the  middle  line,  the  choanae  thus  being  carried  far  back.  Par- 
ietals and  usually  the  frontals  unpaired.  Coracoid  elongate;  no  clavicle. 
Pubis  taking  no  part  in  the  formation  of  the  acetabulum.1  Anterior  feet 
pentadactyl,  posterior  four-toed,  the  fifth  toe  being  represented  only  by  a 
metatarsal.  First  three  digits  clawed. 

SECTION  I.  LONGIROSTRES.  Snout  very  long,  nasals  never  enter- 
ing wall  of  anterior  nares  ;  homodont  dentition.  The  GAVIALID^:,  of  the 
rivers  of  the  Orient,  is  the  only  existing  family.  Rhamphostoma  (Gavialis] 
gangeticus  comes  from  India.  Rhynchosuchus  is  Australian.  The  earlier 
longirostres  have  amphicoelous,  the  later  procoelous,  vertebrae ;  the  group 
first  appears  in  the  lias.  Telosaurus  and  Thoracosaurus  are  fossil. 

SECTION  II.  BREVIROSTRES.  Snout  shorter,  rounded  in  front; 
nasals  forming  part  of  wall  of  anterior  nares  or  close  to  them ;  heterodont 
dentition.  The  ATOPOSAURID^E  were  small  lizard-like  marine  forms  from 
the  upper  Jurassic.  GoniopJwlis  {Diplosaurui),  of  the  family  GONIOPHO- 
LID,E,  comes  from  the  upper  Jurassic  of  Belgium  and  Colorado.  The 
ALLIGATORID^E  have  existed  since  the  cretaceous.  Alligator  contains 
our  North  American  species,  A.  Indus,  as  well  as  other  species  from  South 
America.  These  have  the  edge  of  the  upper  jaw  without  excavation  for 
fourth  tooth  of  the  lower  jaw.  In  the  CROCODILTD^:  such  an  excavation 
occurs.  The  crocodiles  occur  in  the  tropics  of  both  hemispheres,  Crocodilus 
americanus  occuring  in  our  southern  states. 

1  This  is  the  statement  usually  made  ;  but  there  is  some  evidence  to  show  that  the  bone 
usually  called  pubis  is  in  reality  the  prepubis,  the  true  pubis  fusing  after  a  time  with  the 
ischium. 


REPTILES.  329 

ORDER   IX.     PTERODACTYLIA    (PTEROSAURIA, 
ORNITHOSAURIA). 

Extinct  reptiles  adapted  for  flight.  Skin  naked,  bones  of 
limbs  and  vertebrae  hollow  ;  the  caudal  vertebrae  amphiccelous, 
the  others  procoelous ;  three  to  five  vertebrae  fused  to  form  a 
sacrum.  Skull  bird-like  in  shape  and  in  obliteration  of  sutures  ; 
supra-  and  infratemporal  and  preorbital  fossae  present ;  a  bony- 
sclerotic  ring ;  premaxilla  large  ;  teeth,  when  present,  in  alveoli ;: 
quadrate  immovable.  Ribs  two  headed,  sternal  and  abdominal 
rib  present ;  sternum  keeled ;  episternum  and  clavicles  absent. 
The  fore  limbs  have  the  fifth  digit  greatly  elongate,  supporting 
a  membranous  wing  extending  from  the  side  of  the  body  as  in 


FIG.  322.     Restoration  of  Dimorphodon,  after  Woodward. 

the  bats.  The  other  digits  are  normal,  except  the  first,  which  is 
vestigial  or  absent.  The  pelvis  is  weak,  the  hind  limbs  reduced, 
the  feet  five-toed.  The  tail  may  be  long  or  short. 

The  pterodactyls  were  confined  to  the  Jurassic  and  creta- 
ceous periods  of  Europe  and  North  America.  In  habits  they 
were  bird-like  or  bat-like.  Some  of  them  were  sparrow-like  im 
size,  while  in  others  (Pteranodon)  the  skull  was  three  feet  in 
length,  and  the  wings  had  an  expanse  of  twenty  feet.  Casts  of 
the  brain  cavity  show  that  the  brain  was  more  like  that  of  birds- 
than  of  other  reptiles,  especially  in  the  shortness  of  the  hemi- 


330  CLASSIFICATION  OF   VERTEBRATES. 

spheres,  the  widely  separated  optic  lobes,  and  the  presence  of  a 
lateral  projection  (flocculus)  from  either  side  of  the  cerebellum. 

In  the  PTERODACTYLIDJE  teeth  were  present ;  in  the  PTERANODON- 
TID,E  they  were  lacking.  The  pterodactyls  proper  had  a  short  tail ;  in 
others  of  the  family  it  was  longer.  Pteranodon  has  been  found  only  in 
the  middle  cretaceous  of  Kansas,  Dimorpkodon  from  the  lower  lias  of 
Dorsetshire. 

SUB-CLASS   II.     AVES. 

Sauropsida  with  the  body  covered  with  feathers  ;  anterior 
appendages  modified  for  flight  ;  warm  blooded  ;  heart  completely 
divided  ;  only  one  (right)  pertistent  aortic  arch  ;  oviparous. 

The  group  of  birds  is  strongly  marked  off  from  all  other  ver- 
tebrates by  the  feathers.  No  other  animals,  recent  or  extinct, 
are  known  which  had  this  protective  envelope,  and  no  birds  lack 
them.  The  structure  and  development  of  these  characteristi- 
cally avian  features  have  been  described  (p.  94),  but  their 
arrangement  is  of  considerable  importance  in  classification. 

Except  in  a  few  birds  the  feathers  are  not  distributed  evenly 
over  the  body,  but  are  in  distinct  tracts  or  pterylae,  these  being 
separated  by  spaces  (apteria)  with  no  feathers  or  with  only  down- 
feathers.  The  feather  tracts  of  the  wings  bear  the  feathers  of 
flight  which,  according  to  the  part  on  which  they  are  supported, 
receive  different  names.  Those  attached  to  the  hand  are  the 
primaries,  to  the  fore  arm  secondaries,  the  three  proximal  of  the 
fore  arm  feathers  being  the  tertiaries.  Primaries,  secondaries, 
and  tertiaries  together  are  called  the  remiges.  These  remiges 
are  overlaid  above  and  below  by  shorter  feathers,  the  upper  or 
under  wing  coverts.  The  principal  tail-feathers  are  the  rectrices, 
and  these  are  similarly  overlaid  by  the  tail  coverts.  The  feath- 
ers attached  to  the  first  digit  form  the  ala  spuria  or  alula.  Some 
birds  are  completely  naked  when  hatched,  and  are  called  gymno- 
paedes.  Others  (dasypaedes)  are  covered  with  down  upon  their 
escape  from  the  egg,  while  a  few  (pteropaedes)  have  the  contour- 
feathers  developed  before  hatching.  All  of  the  gymnopaedes 
and  some  of  the  dasypaedes  are  fed  by  the  adult,  and  are  conse- 
quently called  altrices  ;  but  most  of  the  latter  group  can  run 
about  at  once  (praecoces). 


BIRDS.  331 

A  marked  sauropsidan  feature  is  found  in  the  scales,  which 
more  or  less  completely  cover  the  feet  and  tarso-metatarsal 
region.  These  may  be  small  and  numerous  (reticulate  tarsus) 
or  in  larger  plates  extending  across  the  tarsus  from  side  to  side 
(scutellate),  or  the  scutes  on  either  side  may  fuse  into  a  contin- 
uous plate  (booted  tarsus).  Various  modifications  and  combi- 
nations of  these  conditions  may  occur.  Dermal  ossifications  are 
entirely  lacking. 

The  skeleton  of  birds  is  usually  characterized  by  lightness 
and  strength,  the  former  being  attained  by  the  hollow  condition 
of  most  of  the  bones,  while  strength  is  the  result  of  frequent 
fusion  of  parts  which  remain  distinct  in  other  vertebrates. 

In  most  birds  the  centra  have  saddle-shaped  extremities. 
Only  in  the  odontormae  were  they  amphiccelous,  while  in  a  few 
water-birds  the  thoracic  vertebrae  are  amphiccelous.  The  num- 
ber of  vertebrae  varies  greatly,  the  variations  in  number  being 
most  noteworthy  in  neck  and  tail.  The  cervical  region  is  long 
and  flexible,  but  in  the  rest  of  the  column  extensive  fusions  of 
vertebrae  take  place ;  the  result  being  that  usually  the  anterior 
thoracic  vertebrae  are  coalesced,  while  the  posterior  thoracics, 
lumbars,  sacral  (one  to  five  in  number),  and  some  of  the  caudals 
unite  to  form  a  synsacrum  more  or  less  intimately  united  with 
the  pelvis.  The  remaining  caudals  in  existing  birds  are  fused 
to  form  a  pygostyle  or  ploughshare  bone  ;  but  in  the  extinct 
saururae  the  caudals  were  distinct  and  decidedly  reptilian  in 
character. 

The_ri^^have^two  Jieads.  Cervical  ribs  are  usually  present, 
but  are  frequently  fused  with  the  vertebrae.  The  thoracic  ribs 
are  divisible  into  vertebral  and  sternal  portions,  the  two  parts 
being  articulated  to  each  other  without  the  intervention  of  a 
cartilaginous  portion.  The  sternal  ribs  join  the  sternum  di- 
rectly. Each  vertebral  rib  bears  on  its  posterior  margin  an 
uncinate  process  which  overlies  the  rib  behind,  thus  giving 
greater  strength  to  the  thoracic  box.1  The  sternum  is  well  ossi- 
fied, and  has  the  shape  of  a  broad  plate,  from  the  ventral  side 
of  which  in  flying  birds  a  strong  keel  or  carina  arises  for  the 
attachment  of  the  muscles  of  flight.  The  presence  or  absence 

1  Uncinate  processes  are  lacking  in  a  few  birds  (Chauna,  Archaopteryx). 


332  CLASSIFICATION  OF   VERTEBRATES. 

of  this  keel  was  formerly  used  as  the  basis  of  division  of  the 
birds  into  ratite  and  carinate  groups.  Its  hinder  edge  may  be 
entire  or  with  one  or  two  deep  notches,  or  there  may  be  a  fora- 
men on  either  side.  An  episternum  is  apparently  present  in 
the  embryo. 


FIG.  323.     Front  and  side  views  of  sternum  of  common  fowl,  from  Huxley. 

The  skull  of  birds  is  noteworthy  for  its  lightness,  and  for 
the  great  extent  to  which  the  fusion  of  certain  of  the  bones, 
especially  those  of  the  cranial  wall,  has  been  carried,  in  all 
except  the  dromaeognaths  and  some  fossil  forms.  A  temporal 
fossa  is  present,  but  the  supratemporal  arcade  is  not  complete. 
Postfrontals  and  postorbitals  are  lacking.  A  jugal-quadrato- 
jugal  arch  extends  from  the  quadrate  to  the  maxilla.  The  max- 
illa is  usually  fixed  ;  but  in  some,  as  in  the  parrots,  it  is  movable, 
and  in  these  cases  its  motion  is  transmitted  by  means  of  the 
palato-pterygoid  and  the  jugal-quadratojugal  arches  to  the  quad- 
rate. The  beak  is  largely  made  up  of  premaxillse,  which  are 
fused  and  have  a  long  frontal  process.  The  orbits  are  placed 
in  front  of  the  brain,  and  except  in  a  few  cases  are  separated  by 
a  bony  interorbital  septum.  Frequently  there  is  an  osseous 
sclerotic  ring.  The  external  nares  are  usually  near  the  orbits. 


BIRDS. 


333 


ar 


s.o 


FIG.  324.  Skull  of  duck,  from  Wiedersheim.  ah,  alisphenoid;  ag,  angulare; 
apf,  foramen  palatinum  anterius  ;  ar,  articulare  ;  bo,  basioccipital ;  bpg,  basipterygoid ; 
bs,  basisphenoid ;  bt,  basitemporal ;  d,  dentary ;  en,  external  nares ;  eth,  ethmoid  ;  eo, 
exoccipital ;  eu,  Eustachian  opening;  fm,  foramen  magnum;  fr,  frontal;  ic,  fora- 
men for  internal  carotid ;  j,  jugal ;  tc,  lachrymal  ;  mx,  maxilla ;  mxp,  palatine 
process  of  maxillary;  n,  nasal;  npjc,  nasal  process  of  premaxillary ;  /,  parietal;  pg, 
pterygoid ;  pi,  palatine;  pn,  posterior  nares;  ps,  presphenoid ;  px,  premaxilla;  q, 
quadrate  ;  qj,  quadratojugal ;  so,  supraoccipital ;  sq,  squamosal ;  ty,  tympanic  cavity ; 
v,  vomer ;  //,  V,  IX,  X,  XII,  openings  for  corresponding  nerves. 


334 


CLASSIFICATION  OF   VERTEBRATES. 


FIG.  325.  Illustrating  the  hinging  and  movements  of  the  maxilla  and  other 
bones  in  the  birds;  modified  from  Boas,  y,  jugal;  A7,  nares;  O,  orbit;  P,  palatine; 
PTt  pterygoid ;  Q,  quadrate. 

The  quadrate  is  freely  movable.     In  the  floor  of  the  skull  appear 
three  bones  which  in  part  replace  the  parasphendid  of  the  ich- 

thyopsida.  These  are,  behind,  a 
pair  of  basitemporals  (fused  in 
the  adult),  and,  in  front,  a  me- 
dian rostrum.  The  rami  of  the 
lower  jaw  are  anchylosed  at  the 
symphysis. 

The  relations  of  palatines, 
pterygoids,  and  vomers  show 
many  variations,  and  have  been 
used  as  a  basis  of  classification 
of  birds.  Although  this  classi- 
fication has  not  obtained  accept- 
ance, the  conditions  are  fre- 
quently used  as  characters,  and 
may  be  described  here. 

In  the  dromaeognathous  skulls 
the  vomers  form  a  broad  bone 
separating  the  rostrum  from  the 
palatines  (Fig.  326),  and  occa- 
sionally from  the  pterygoids.  In 
all  others  the  palatines  and 
pterygoids  articulate  with  the 
rostrum,  but  they  differ  in  other 
respects.  In  the  desmognathous 

type  the  vomer  is  more  or  less 

FIG.    726.       Skull  of  ostrich  (dro-  -,.  i  ••.      .-,  -n 

rudimentary,  while  the  maxillo- 

mseognathous  type).     //,  palatine;  //,  J 

pterygoid;  r,  rostrum;  v,  vomer.  palatine  processes  unite   in   the 


BIRDS, 


335 


median  line  forming  a  bony  roof  across  the  palate  (Fig.  324). 
In  the  schizognathous  skull  there  is  a  gap  between  the  max- 
illopalatine  processes,  and  another  between  them  and  the  vomer 
when  the  latter  is  present,  the  latter  bone  being  pointed  in 
front.  In  the  aegithognathous  forms  the  vomer  is  truncate  in 
front  and  cleft  behind,  the  rostrum  being  embraced  between 
its  forks.  In  the  saurognathous  skull  the  vomer  is  paired. 

The  hyoid  arch  is  but  slightly  devel- 
oped ;  the  first  branchials,  on  the  other 
hand,  may  be  large.  These  two  form 
a  Y-shaped  structure,  the  stem  consist- 
ing of  an  anterior  os  entoglossum  (paired 


~*--     — — 

FIG.  327.  Skull  of  quail,  schiz- 
ognathous  type.  /*/,,  maxillopala- 
tines ;  PT,  pterygoid  ;  Q,  quadrate  ; 
J?,  rostrum  ;  V,  vomer. 


FlG.  328.  Sternum  and  pectoral  girdle 
of  CasuariuS)  after  Parker.  <:,  coracoid; 
cl,  clavicle ;  pc,  procoracoid ;  s,  scapula. 
Cartilage  dotted. 


in  origin  and  possibly  representing  the  hyoid  arches)  and  a 
posterior  basihyal  from  which  arise  a  pair  of  cornua  (ist  bran- 
chials), which  are  usually  made  up  of  several  segments.  Ex- 
tending backwards  from  the  basihyal,  between  the  cornua  is  a 
'urohyal,'  in  reality  a  basibranchial. 

The  pectoral  girdle  is  well  developed  except  in  the  flightless 
birds,  and  in  some  moas  apparently  it  was  entirely  absent.  The 
coracoids  are  large,  the  procoracoids  rudimentary  or  absent.  In 
most  ratite  forms  the  clavicles  are  absent ;  in  all  other  birds  they 


336 


CLASSIFICATION-  OF   VERTEBRATES. 


.are  well  developed,  their  proximal  ends  being  usually  fused,  thus 
giving  rise  to  the  furcula  or  '  wish-bone.'  The 
scapula  is  either  at  right  angles  to  the  coracoid 
(carinate  birds)  or  in  the  same  straight  line 
with  it  (ratite  birds). 

In  the  fore  limb  the  distal  elements  show 
great  reduction,  the  carpals  being  reduced  by 
fusion,  while  the  hand  has  but  three  digits  in 
the  adult  (four  in  the  embryo),  these  appar- 
ently 2,  3,  and  4  of  the  normal  hand.  The 
metacarpals  usually  show  more  or  less  com- 
plete fusion. 

The  pelvis  is  large  and  characteristic.  Its 
bones  are  fused  in  all  except  Archceopteryx, 
and  the  acetabulum  is  perforated.  The  ilium 
is  elongated  in  front,  and  is  fused  with  all  the 
vertebrae  of  the  synsacrum  (p.  331).  The  pu- 
bis  1  and  ischia  are  directed  backwards,  and  are 
usually  without  symphysis.  The  distal  end  of 
FIG.  329.  Devel-  the  ischium  may  fuse  either  with  the  ilium 
of  wing  (most  birds)  or  with  the  pubis. 

In  the  hind  limb  the  noticeable  features  are 
the  great  reduction  of  the  fibula,  the  fusion  of 
the  proximal  tarsals  with  the  tibia  (tibio-tarsus), 
and  the  distal  tarsals  to  the  metatarsals  (tar- 
so-metatarsus),  the  ankle  joint  thus  being  in- 
tertarsal.  In  the  foot  but  four  toes  occur  in 
the  adult  (five  in  certain  embryos)  these  be- 
lt is  interest- 


opment 
skeleton 
{Sterna  wilsoni}, 
after  Leighton.  A, 
early;  B,  late,  c, 
carpales ;  cl,  claw ; 
.me,  metacarpals ; 
.r,  radius;  re,  radi- 
:ale ;  u,  ulna  ;  ue,  ul- 
aiare;  II- V,  digits. 


ing   I,   2,   3,  4. 

ing  that  usually  the  phalanges 
increase  in  number  from  the 
-first  to  the  fifth  toe  in  the 
^rder  2,  3,  4,  5,  as  in  many 
lizards. 

1  The  bone  called  pubis  corresponds 
-to  the  postpubis  of  dinosaurs  ;  the  pre- 
pubis  being  represented  wholly  or  in  part 
in  the  pectineal  process  which  occurs  in 
many  birds  (Fig.  331). 


FlG.  330.  Pelvis  of 
raven,  from  the  side.  A, 
acetabulum;  //.,  ilium  ;  75, 
ischium ;  O,  obturator  foramen ;  /*, 
pubis. 


BIRDS. 


337 


All  existing  birds  are  toothless,  but  the  dental  ridge  (p.  1 9) 
is  formed  in  the  embryo  of  at  least  a  few  forms.  In  several 
fossil  birds  teeth  were  present,  either  in  grooves  (Archceopteryx, 


FIG.  331.     Pelvis  of  Apteryx,  after  Marsh,  from  Wiedersheim.     a,  ace- 
tabulum;   /'/,  ilium  ;  is,  ischium;  pl,  postpubis;  /,  prepubis. 

Hesperornis)   or  in  alveoli   (IcJitJiyornis).     Many  modern   birds 

have  the  horny  sheath  of  the  beak  produced  into  horny  tooth-like 

processes,  which  in  many  cases  are  supported  by  corresponding 

elevations  of  the  bone.     The  tongue 

is  well  developed  and  protrusible,  and 

exhibits  many  modifications    in  form. 

In    most    birds    the    oesophagus   is   of 

the    same    size    throughout,     but    in 

grain-eating  birds  and  birds  of  prey  it 

has    an    enlargement    or    crop    which 

serves  as  a  reservoir  of  food,  and  in 

many   cases    is    glandular,   and    hence 

plays  a  part  in  digestion.     The  stom-          FIG.  332.   Transverse  sec- 

,        T  .   .          f    A  v    •    •  tion   through  the  beak  of  an 

ach  always  consists   of  two   divisions,     embryo  ^  after  Rb-se     ^ 
an     anterior    glandular     stomach     or     dental  ridge ;  bone,  black, 
proventriculus    and    a   second    muscu- 
lar stomach  or  gizzard,  the  muscles  of  the  latter,  which  radiate 
from    a    tendinous    centre    on    either    side,   being    best    devel- 
oped  in  the   grain-eating  forms.     A  <  pyloric  stomach'  occurs 


338 


CLASSIFICATION  OF   VERTEBRATES. 


TR 


in  a  few  birds  between  the  gizzard  and  the  intestine.  The 
length  and  coiling  of  the  intestine  varies  greatly,  the  large 
intestine  usually  being  short  and  straight.  There  are  usually 
two  caeca  at  the  junction  of  ileum  and  colon.  The  cloaca  re- 
ceives on  its  dorsal  wall  the  urinary  and  genital  ducts,  and 
farther  back,  in  the  young  of  all,  there  is  developed  a  sac,  the 
bursa  Fabricii,  of  unknown  function.  It  usually  degenerates  in 
the  adult  (Fig.  131). 

The  liver  is  usually  two  lobed,  and  there  are  two  or  three 
bile  ducts.  A  gall  bladder  usually  occurs.  The  pancreas  is 
large  and  compact,  and  lies  in  the  duodenal  loop.  The  spleen 
is  small. 

The  trachea  is  usually  straight,  but  it  may  be  folded  or  con- 
voluted, the  convolutions  lying  beneath  the  skin  of  the  abdomen 

or  inside  the  keel  of  the  breast 
bone.  The  tracheal  rings  are 
ossified.  The  syrinx  (p.  29)  is 
usually  formed  by  trachea  and 
bronchi,  but  it  may  be  purely 
bronchal  or  entirely  tracheal  in 
character.  The  lungs  do  not 
lie  free  in  the  coelom,  but  are 
bound  to  its  dorsal  wall  by 
cellular  tissue.  The  air  sacs 
are  usually  large,  and  it  is  by 
changes  in  the  size  of  these 
more  than  by  alterations  in  the 
volume  of  the  lungs  themselves 
that  inspiration  and  expiration 
are  effected. 

The  brain  is  characterized  by  its  compact  form  and  the 
large  size  of  the  non-convoluted  cerebrum,  which  reaches  back 
to  meet  the  cerebellum,  thus  forcing  apart  the  optic  lobes.  The 
cerebrum  is  largely  made  up  of  the  corpora  striata,  and  a  small 
corpus  callosum  is  present.  The  olfactory  lobes  are  small 
(much  larger  in  extinct  birds).  The  cerebellum  has  a  median 
vermis,  showing  an  arbor-vitae  in  section,  and  a  pair  of  small 
lateral  floccular  lobes  ;  a  pons  is  lacking.  The  twelve  cranial 


TR 


FIG.  333.  Diagrammatic  section  of 
•syrinx,  after  Boas.  f£,  walls  of  drum; 
S,  bridge;  BR,  bronchus;  TR,  trachea. 


339 


r.AHJS. — 


FlG.  334.  Air  sacs  of  duck,  from  Wiedersheim.  Aa,  anonymous,  Ap,  pulmonary 
artery ;  C,  cervical  sac  ;  Cd,  coracoid ;  D,  intestine  ;  Dtha,  diaphragm  ;  F,  furcula ; 
H,  heart ;  lAbdS,  left  abdominal  sac ;  led,  Ics,  ligamentum  coronarium  hepatis ;  Lgy 
lung  ;  Ifcd,  lig.  coraco-f  urcularis  ;  IL,  liver  ;  Ish,  suspensor  of  liver  ;  M,  stomach ;  Oe , 
oesophagus ;  P,  pectoralis  ;  /,  pectoral  sac ;  pa,  artery  to,  pv,  vein  from,  pectoralis ; 
;•/,,  liver  ;  rAbdS,  right  abdominal  sac;  S,  subclavian  ;  s,  partition  between  the  ante- 
rior diaphragmatic  and  the  supracoracoid  sac ;  s1,  between  anterior  and  posterior  dia- 
phragmatic sacs ;  T,  trachea ;  z>,  anterior  wall  of  supracoracoid  sac  ;  Va,  anonymous 
vein  ;  *,  entrance  of  bronchi  into  lung  ;  t,  anterior,  tt,  posterior  diaphragramatic  sac. 


340 


CLASSIFICATION  OF   VERTEBRATES. 


nerves  are  distinct  in  origin.  A  characteristic  is  the  double 
condition  of  the  sympathetic  in  the  region  of  the  neck,  one 
portion  following  the  vertebrarterial  canal,  the  other  the  carotids. 
The  nostrils,  except  in  Apteryx,  are  near  the  orbits.  Con- 
nected with  the  olfactory  organ  is  a  nasal  gland,  usually  situated 
in  the  frontal  bone,  its  duct  emptying  into  the  respiratory 
chamber.  The  eyes  are  large  and  highly  developed,  and  are 
spherical  behind,  obtusely  conical  in  front.  Except  in  Apteryx 
there  is  developed  a  peculiar  fold,  the  pecten  or  marsupium,  which 
is  vascular,  and  projects  into  the  posterior  chamber  in  the  line 
of  the  choroid  fissure.  The  nictitating  mem- 
brane is  large,  transparent,  and  is  moved  by 
two  special  muscles,  the  quadratus  and  the 
pyramidalis.  The  associated  Harderian  gland 
is  large,  the  lachrymal,  at  the  external  angle 
of  the  orbit,  being  small.  The  eye  muscles 
proper  are  small ;  and  the  eyeball  is  some- 
what limited  in  its  motions,  this  being  com- 
pensated by  the  flexibility  of  the  neck.  The 
ear  has  a  large  semicircular  canal ;  and  the 
lagena  is  long  and  slightly  coiled,  its  distal 
end  being  somewhat  expanded.  It  recalls 
the  cochlea  of  the  mammals,  but  a  Corti's 
organ  is  lacking.  The  tympanic  cavity  sends 
prolongations  into  the  surrounding  bones, 
one,  the  siphoneum,  penetrating  the  lower 
jaw.  The  tympanum  is  crossed  by  the  slender  columella,  which 
bears  at  its  membranal  end  a  discoid  stapedial  plate.  The 
external  ear  is  surrounded  by  a  circle  of  feathers ;  and  in  some 
birds  (e.g.,  owls)  these  may  be  moved,  like  a  valve,  by  appro- 
priate muscles, 

The  heart  is  completely  divided  into  right  and  left  halves; 
and  the  dorsal  aorta  is  supplied  only  by  the  right  aortic  arch, 
the  left  of  the  normal  pair  being  converted  into  the  innominate 
artery.  There  is  no  mixture  of  arterial  and  venous  blood  in  the 
heart.  The  blood  returns  to  the  heart  usually  by  two  precavae 
and  a  single  postcava,  these  emptying  separately  into  the  right 
auricle  in  which  the  sinus  has  become  merged.  There  is  no 


FIG.  335.     Brain 
of  bird. 


34i 

renal  portal  system.  Characteristic  of  birds  is  an  arterial  plexus 
beneath  the  skin  of  the  ventral  side,  which  becomes  greatly 
enlarged  at  the  time  of  incubation.  The  red  blood  corpuscles 
are  oval  and  nucleated. 

The  permanent  kidneys  are  metanephridia.  Usually  they 
consist  of  three  lobes,  each  lobe  lying  in  a  cavity  bounded  by 
the  vertebrae  and  the  transverse  processes.  Frequently  the 
kidneys  meet  and  even  fuse  posteriorly.  The  ureters  open 
separately  in  the  cloaca.  No  urinary  bladder  occurs.  The 
urine  is  white  and  semi-solid. 

The  left  ovary  is  never  functional,  and  it  and  its  duct  are 
usually  aborted.  The  right  ovary  is  strongly  lobulated  on  ac- 
count of  the  large  size  of  the  eggs.  The  corresponding  ovi- 
duct has  a  large  funnel-shaped  opening,  and'  is  divided  into 
three  regions,  the  middle  of  which  is  glandular  and  furnishes 
the  white,  while  the  posterior  is  both  muscular  and  glandular 
and  secretes  the  egg-shell.  From  the  fact  that  the  egg  remains 
some  time  in  the  latter  division,  this  is  sometimes  spoken  of  as 
the  uterus.  The  testes  are  usually  equally  developed  ;  they  lie 
in  front  of  the  kidneys,  and  the  vasa  deferentia  have  a  convo- 
luted course,  opening  separately  into  the  cloaca.  A  copulatory 
organ  is  usually  absent.  In  the  ostrich  there  is  a  solid  retrac- 
tile penis  like  that  of  alligators  and  turtles,  while  a  few  other 
ratites  and  aquatic  birds  have  the  ventral  wall  of  the  cloaca 
thickened,  with  a  median  groove  which  serves  as  a  sperm  duct. 
Secondary  sexual  characters  are  common.  The  male  may  be 
either  larger  or  (more  rarely)  smaller  than  the  female.  Fre- 
quently he  is  distinguished  by  brighter  colors,  by  the  develop- 
ment of  certain  feathers,  etc. 

The  eggs  are  very  large,  and  are  incubated  by  the  parents  ; 
the  period  of  incubation  varying  from  eleven  days  to  seven 
weeks  (ostrich).  The  nest-building  habits  vary  greatly. 

The  development  of  different  birds  shows  few  and  unim- 
portant variations,  and  the  history  of  the  common  chick  is  well 
known.  The  early  phases  of  segmentation  are  passed  through 
before  the  egg  is  laid.  This  segmentation  affects  at  first  only 
a  small  portion  of  the  upper  surface  of  the  yolk  (i.e.,  is  mero- 
blastic).  The  resulting  blastoderm  is  several  cells  in  thickness, 


342  CLASSIFICATION  OF   VERTEBRATES. 

a  layer  of  superficial  ectoderm  cells,  and  beneath  this  the  lower 
layer  cells  of  undifferentiated  mesoderm  and  entoderrn.  The 
blastoderm  next  exhibits  two  areas,  —  a  translucent  central  area 
pellucida  and  a  marginal  area  opaca.  At  the  edge  of  the  blas- 
toderm there  now  appears  an  elongate  depression,  the  primitive 
streak  (an  extremely  modified  blastopore),  the  axis  of  which, 
corresponding  to  the  future  axis  of  the  embryo,  lies  at  right 
angles  to  the  major  axis  of  the  egg.  In  front  of  this  arise  a  pair 
of  medullary  folds  enclosing  the  medullary  groove,  the  hinder 
ends  of  the  folds  embracing  the  anterior  end  of  the  primitive 
streak. 

While  the  embryo  is  thus  being  outlined  the  blastoderm  in- 
creases in  size,  and  soon  becomes  differentiated  into  embryonic 
and  extraembryonic  portions,  the  former  giving  rise  to  the  whole 
of  the  embryo,  the  other  to  a  cellular  yolk  sac  which  eventually 
embraces  the  whole  yolk.  At  the  edge  of  the  embryonic  area 
arises  the  amniotic  fold  which  closes  in  over  the  embryo  from 
all  sides,  thus  enclosing  it  in  a  double-walled  sac,  the  inner 
layer  being  the  amnion,  the  outer  the  serosa.  While  the  am- 
nion  is  being  closed  in,  the  embryo  begins  to  be  cut  off  from 
the  yolk,  at  last  only  a  narrow  yolk  stalk  connecting  the  two. 
The  allantois  grows  out  from  the  alimentary  tract  behind  the 
yolk  stalk.  At  its  base  it  is  small,  but  it  expands  distally  into 
a  large  vesicle.  Both  yolk  sac  and  allantois  have  blood-vessels 
developed  in  them,  and  form  important  organs  of  nutrition  in 
the  broader  sense.  The  blood-vessels  of  the  yolk  sac  appear 
first  as  outgrowths  from  the  omphalomesaraic  arteries  and  veins 
in  the  area  pellucida  ;  but  they  gradually  extend  over  the  area 
opaca,  branching  and  forming  a  plexus,  the  function  of  which  is 
to  take  up  the  yolk  and  carry  it  into  the  circulation.  The 
allantoic  circulation  is  respiratory  in  character.  Its  vessels  are 
outgrowths  from  the  anterior  abdominal  vessels  of  the  non- 
allantoidan  vertebrates.  The  allantois  also  serves  as  a  reservoir 
of  urinary  waste. 

In  the  general  growth  of  the  embryo  the  most  striking 
feature  is  the  close  similarity,  until  a  late  stage,  with  the  rep- 
tiles. The  gill  slits  —  three  or  four  in  number  —  never  bear 
gills,  and  the  appendages  in  the  early  stages  are  distinctly  paw- 


BIRDS. 


343 


or  flipper-like.  As  the  yolk  is  absorbed,  the  yolk  sac  is  drawn 
into  the  body  cavity,  and  the  abdominal  walls  close.  Then  the 
shell  is  broken,  in  most  birds  by  means  of  a  calcareous  or 
horny  growth  at  the  tip  of  the  upper  jaw  (egg-tooth),  and  the 
young  begins  its  free  life. 

As  far  as  is  at  present  known,  birds  appeared  (A  rchceopteryx, 
Laopteryx)  in  the  Jurassic.  In  the  cretaceous  the  genera  IcJithy- 
ornis  and  Hesperornis  are  found,  while  in  the  tertiary  the  forms 
are  more  numerous,  although  at  all  times  fossils  belonging  to 
the  group  are  rare. 

The  order  of  birds  is  so  uniform  in  its  structural  features 
that  it  is  difficult  to  find  important  characters  to  differentiate 
the  twelve  thousand  known  species  into  convenient  groups.  As 
a  result,  ornithologists  have  raised  a  number  of  minor  groups 
into  so-called  orders,  which  are  scarcely  of  family  rank,  if  we 
are  to  accept  the  rules  in  vogue  in  other  groups  of  vertebrates. 
The  group  by  most  authors  is  sub-divided  into  Ratitae  and 
Carinatae,  divisions  based  upon  the  presence  or  absence  of  a 
keel  to  the  sternum  ;  but  these  divisions  are  artificial,  and  do  not 
indicate  the  phylogeny  of 
the  forms  concerned. 

ORDER  I.    SAURUR^E 
(ARCH^EORNITHES). 

Extinct  birds  with 
elongate  tail  consisting 
of  many  vertebrae  ;  up- 
per jaw  with  teeth  (low- 
er unknown)  ;  vertebrae 
amphicoelous  ;  feathers 
of  the  normal  type,  those 
of  the  tail  in  pairs,  a 
pair  to  each  vertebra. 
Only  two  specimens  are 
known,  both  coming 
from  the  lower  Jurassic 
slates  of  Bavaria.  These  belong  to  the  genus  Archceopteryx, 
but  may  represent  two  distinct  species.  One  is  about  the  size 


FIG.  336.     Restoration  of  Archccopteryx, 
after  Pycraft. 


344  CLASSIFICATION  OF   VERTEBRATES. 

of  a  crow,  the  other  considerably  larger.  Laopteryx,  known 
from  a  few  fragments  from  the  Jurassic  of  Wyoming,  may  be- 
long here. 

ORDER   II.     ODONTORM^:. 

Extinct  carinate  birds  with  normal  avian  tail  (pygostyle)  ; 
teeth  thecodont ;  presacral  vertebrae  amphicoelous ;  quadrate 
with  a  single  articular  facet ;  rami  of  lower  jaw  united  by 
cartilage. 

To  this  order  belong  a  few  birds  arranged  in  the  genera 
Ichthyornis  and  Apatornis,  pigeon-like  in  size,  found  in  the 
middle  cretaceous  of  Kansas  and  Colorado.  They  had  very 
large  skulls,  strong  wings,  and  small  legs,  while  the  succession 
of  the  teeth  was  vertical  as  in  the  dinosaurs.  This  and  the  fol- 
lowing order  are  frequently  united  as  odontornithes  or  toothed 
birds. 

ORDER   III.     ODONTOHOLC^E. 

Extinct  ratite  birds  with  teeth  in  alveolar  grooves  ;  vertebral 
centres  saddle-shaped ;  quadrate  with  one  articular  facet ;  skull 
dromaeognathous ;  rami  of  lower  jaw  united  by  cartilage;  wing 
reduced,  only  the  humerus  retained. 

The  birds  belonging  to  this  order  occur  in  the  same  beds 
as  do  the  odontormae.  In  general  appearance  they  were  some- 
what like  grebes.  The  cranial  bones  were  firmly  united,  the  pre- 
maxillary  bone  was  without  teeth,  while  the  teeth  of  the  maxillae 
and  lower  jaw  had  a  lateral  succession  as  in  the  pythonomorphs. 
There  was  no  true  pygostyle,  but  the  caudal  vertebrae  were 
broadly  expanded,  forming  a  paddle-like  tail,  only  a  few  of  the 
distal  bones  being  fused.  The  clavicles  were  not  united  into 
a  wish-bone ;  the  acetabulum  resembled  that  of  the  crocodiles ; 
ilia,  ischia,  and  pubes  were  not  united  posteriorly ;  and  the  feet 
were  apparently  fitted  for  swimming  only.  Here  belong  Hes- 
perornis  and  Lestornis.  H.  regalis  was  about  six  feet  long ; 
L.  crassipes  considerably  larger. 


BIRDS. 


345 


FIG.  337-     Restoration  of  Hesperornis,  after  Marsh. 


ORDER   IV.     EURHIPIDUR^:. 

Toothless  birds  with  pygostyle  and  saddle-shaped  centra  to> 
the  presacral  vertebrae.  The  rami  of  the  lower  jaw  are  firmly 
united*  To  this  order  belong  all  living  and  many  extinct  birds. 


\ 


SUB-ORDER  i.     DROM^EOGNATHI. 


Eurhipidurine  birds  with  dromasognathous  palate  ;  ischium  and  ilium-, 
not  united  behind  ;  sternum  either  ratite  or  carinate  ;  wings  rudimentary  or 
of  no  use  in  flight. 

SECTION  I.  STRUTHIONES.  Large  ratite  birds  with  elongate  hind 
legs  and  neck  ;  bill  broad  at  base ;  mouth  deeply  split ;  toes  three  or  two. 
Restricted  to  the  southern  hemisphere.  The  STRUTHIONID^:,  containing 
the  single  species  Struthio  camelus,  the  ostrich  of  Africa,  has  but  two  toes. 
The  South  American  RHEID^:,  which  have  three  toes,  contain  the  nandu,. 


346 


CLASSIFICATION-  OF   VERTEBRATES. 


Rhea  americana,  the  feathers  of  which  are  familiar  in  feather  dusters. 
The  CASUARID^E  of  the  Oriental  region  have  three  toes  and  a  helmet-like 
development  on  the  head.  The  family  contains  the  emeus  (Dromaius)  and 
the  cassowaries  (Casuarius).  A  fourth  family,  the  DINORNITHID^E,  the 
extinct  moas  of  Australia  and  New  Zealand,  were  birds  of  gigantic  size. 

SECTION.   II.     yEPIORNITHES.      Extinct  ratite  birds  of  large  size, 
formerly  inhabiting  Madagascar.     ^Epiornis. 


FIG.  338.     South  American  Ostrich,  Rhea  americana,  from  Liitken. 


SECTION  III.  APTERYGES.  Dromaeognathous  ratite  birds  with 
rudimentary  wings ;  no  clavicle ;  four  toes  ;  vomer  united  to  palatines  and 
pterygoids  ;  bill  long,  nostrils  near  the  tip.  Four  existing  species  of  kiwi, 
belonging  to  the  genus  Apteryx,  inhabit  New  Zealand. 

SECTION  IV.  CRYPTURI.  Dromaeognathous  carinate  birds  with 
clavicle  and  functional  wings.  About  50  species  from  Central  and  South 
America.  Crypturus,  Rhynchotus,  Tinamus. 

SECTION  V.  GASTORNITHES.  Extinct  carinate  birds  from  the 
•eocene  of  France  and  Belgium.  Gastornis.  ?  Diatryma  from  New 
Mexico. 


BIRDS. 


347 


SUB-ORDER  2.     IMPENNES. 

Aquatic  birds  with  short,  paddle-like  wings  used  only  for  swimming. 
Pterylae  and  apteria  not  differentiated ;  no  differentiated  remiges ;  dorsal 
vertebrae  opisthocoelous  and  movable; 
synsacrum  poorly  developed ;  skull 
schizognathous ;  uncinate  processes 
not  anchylosed  to  ribs  ;  pollex  absent ; 
pubis  not  united  to  ischium  behind  ; 
four  toes  ;  feet  plantigrade.  The  pen- 
guins, of  which  there  are  several  gen- 
era (Aptenodytes,  Spheniscus,  etc.)  are 
confined  to  the  colder  portions  of  the 
southern  hemisphere.  They  are  flight- 
less, but  use  their  wings,  which  are 
covered  with  scale-like  feathers,  as 
paddles  in  swimming.  They  feed 
upon  fish  and  shell-fish,  and  make 
their  nests  upon  uninhabited  islands. 
Palceeudyptes  occurs  in  the  eocene  of 
New  Zealand. 


.  339.     Penguin,  Aptenodytes 
longirostris,  after  Liitken. 


SUB-ORDER  3.     EUORNITHES. 

Non-dromaeognathous    birds    with 
(usually)  saddle-shaped    centra    to  the 
dorsal    vertebrae;    distal   caudal   verte- 
brae united  to  a  pygostyle ;  quadrate  with  two  articular  facets  ;  ilium  and 
ischium    united    behind,    enclosing    an    iliosciatic    foramen;    pollex   free; 
pterylae  and  apteria  differentiated. 

To  this  sub-order  belong  the  great  majority  of  living  birds,  over  twelve 
thousand  in  number.  They  are  usually  sub-divided  into  a  number  of  groups 
commonly  regarded  as  '  orders,'  but  which  are  only  of  family  rank,  the  so- 
called  families  being  equivalent  to  genera  in  other  groups  of  vertebrates. 
Reference  must  be  made  to  special  works  on  ornithology  for  details,  as 
space  will  only  allow  mention  of  families  here,  with  such  features  as  will 
allow  of  correlation  of  other  works  with  the  system  here  adopted. 

SECTION  I.  DESMOGNATH/E.  Birds  with  desmognathous  pala- 
tine structure  (p.  334).  The  STEGANOPODES  are  strong  flying,  web-footed 
birds,  in  which  all  four  toes  are  directed  forwards,  while  basipterygoid 
processes  are  lacking.  The  tropic  birds  (Phaethoti)  have  all  the  toes  con- 
nected by  a  web,  while  in  the  frigate  birds  (Fregata}  the  web  is  scarcely 
developed.  The  pelicans  (Pelecanus},  with  twenty-four  tail-feathers,  are 
characterized  by  the  enormous  pouch  connected  with  the  lower  jaw.  The 
gannets  (Sula),  the  cormorants  (Phalacrocorax},  and  the  darters  (Anhinga} 
also  belong  here.  The  CHENOMORPH^E  have  three  toes  directed  forwards, 


348 


CLASSIFICATION  OF   VERTEBRATES. 


and  in  most  cases,  as  in  the  ducks  (Anas),  geese  (Anser),  swans  (Cygnus), 
and  flamingoes  {Phenicopterus},  webbed  and  fitted  for  swimming,  while 
in  the  screamers  (Anhima)  the  web  is  lacking.  The  young  when  hatched 
are  feathered,  and  able  to  feed  themselves.  The  HERODII  includes 
altrical  forms  (p.  330),  in  which  the  legs  are  very  long,  the  toes,  of  which 
three  are  directed  forwards,  are  usually  without  webs,  and  these  birds, 
like  the  grallae  of  .the  schizognathous  section,  are  wading  forms.  The 
various  species  of  Ibis,  the  spoonbills  (Platalea),  storks  (Ciconid),  herons 
(Ardea,  Herodias*},  and  bitterns  (Botaurus},  are  familiar  examples.  The 
ACCIPITRES  (Raptores)  are  recognized  by  their  hooked  bill  and  claws,  the 

toes,  three  of  which  are  di- 
rected forwards,  being  with- 
out webs.  There  is  no  ba- 
sipterygoid  process,  and  the 
young  are  altrical  in  char- 
acter. The  hooked  beak 
is  shared  by  the  parrots, 
but  the  toes  at  once  dis- 
tinguish the  two  groups. 
The  birds  of  prey  include 
the  vultures  and  buzzards 
(Cathartes,  Gyps,  Sarcor- 
hainphns},  the  eagles 
{A guild],  hawks  (Buteo,  Ac- 
cipiter),  and  falcons  {Falco], 
forms  which  are  closely  alike 
in  structure  and  differ  con- 
FIG.  340.  Wood-duck,  Aix  sponsa,  from  siderably  from  the  nocturnal 

Tenney,  after  Audubon.  owls  *    {Strix,  Bubo,    Scops, 

etc.),  which  compose  the  rest 

of  the  family.  The  COCCYGOMORPH^E,  as  a  rule,  have  three  toes  directed 
forwards,  but  in  the  cuckoos  and  toucans  the  first  and  fourth  toes  are 
turned  backwards,  while  in  the  colies  (Colius]  all  four  toes  are  directed 
forwards.  In  all  the  rostrum  is  movable.  Typical  genera  are  the  plan- 
tain-eaters {Musophaga),  the  cuckoos  (Cuculus,  Geococcyx},  the  night-hawks 
{Caprimulga,  Chordediles),  the  rollers  (Coracias],  bee-eaters  (Merops], 
motmots  {Momotus),  todies  (Todus],  kingfishers  {Halcyon  and  Alcedo], 
the  hornbills  (Bitceros},  hoopoes  {Upupd},  puffbirds  (Monasa,  Bucco},  tou- 
cans (Rkampkastos),  and  honey  guides  {Indicator}.  The  TROGONID^E 
are  characterized  by  having  toes  one  and  two  directed  backwards.  The 
STEATORNITHID^E  resemble  the  rollers  in  many  of  their  characters,  but 
they  differ  from  them,  as  from  all  desmognaths  except  the  parrots,  in 
the  opisthocoele  character  of  the  vertebrae.  Three  toes  are  directed 
forward.  The  oilbird  (Steatornis  caripensis]  of  South  America  is  the 
1  The  owls  may  be  more  nearly  related  to  Coracias  than  to  the  Accipitres. 


BIRDS. 


349 


only  species.  The  parrots  or  PSITTACI  agree  with  the  last-mentioned  family 
in  the  vertebral  centra  and  movable  rostrum,  but  have  the  first  and  fourth 
toes  turned  backwards,  while  the  beak  is  hooked.  Conurus,  Psittacus, 
Cacatna,  Trichoglossus  are  typical  genera. 


FIG.  341.     Carolina  paroquet,  Conurtts  carolinensis,  from  Tenney,  after  Wilson. 

SECTION  II.  SCHIZOGNATH/E.  Birds  in  which  there  is  a  schizog- 
nathous  palatal  structure  (p.  355),  and  the  vomer  is  narrowed  and  acute  in 
front.  The  families  to  some  extent  parallel  those  of  the  desmognathae  in 
appearance  and  habits.  The  CECOMORPHiE  are  swimming-birds,  in  which 
the  feet  are  webbed,  three  toes  pointing  forwards,  and  the  external  nostrils 
are  prolonged  backwards  as  a  fissure.  The  family  includes  the  grebes 
(Colymbus  and  Podiceps),  loons  (Urinator),  sun-grebes  (Heliornis),  auks 
{A lea),  guillemots  (Uria),  gulls  (Larus).  terns  (Sterna),  and  skuas  (Ster- 
corarius).  The  TUBINARES,  including  the  albatrosses  (Diomeded),  petrels 
(Procellarid) ,  and  fulmars  (Fulniarus),  closely  resemble  the  cecomorphae, 
except  in  the  tubular  nostrils.  They  are  oceanic  in  their  habitat.  The 
GRALL^E  are  long-legged  wading  birds  in  which  the  toes  (three  directed 
forwards)  are  not  usually  webbed.  The  nostrils  are  either  as  in  the  ceco- 
morphae, or  they  are  closed  behind  by  a  rounded  edge.  The  pratincoles, 
plovers  (Charadrius),  Jacana,  snipes  (Scolopax),  cranes  (Grus),  and  rails 
(Rallus),  and  their  allies,  are  littoral  forms,  while  the  bustards  (Otis)  have 
lost  their  wading  habits  and  are  truly  terrestrial.  The  OPISTHOCOMI  of 
South  America,  like  all  the  remaining  schizognathous  families  have  three 
toes  directed  forwards.  In  general  appearance  the  single  species  recalls 
the  Gallinae,  but  differs  in  the  absence  of  the  basipterygoids,  the  union  of 


350 


CLASSIFICATION  OF   VERTEBRATES. 


lachrymals  to  the  rostrum,  etc.     The  GALLING  (Rasores,  Alectoromorphas) 

includes  the  quail  (Coturnix),  partridge  (Perdix),  grouse  (Tetrao,  Bonasa), 

jungle-fowl,  including  our  domestic  fowl 
(Callus),  pheasants  (Phasianus,  Thau- 
malea),  turkeys  (Meleagris),  peafowl 
(Pavo) .  These  have  the  hallux  rudimen- 
tary and  elevated  above  the  other  toes 
and  two  carotid  arteries.  The  COLUMB^E 
(Pullastrae)  have  usually  two  carotids 
and  the  hallux  well  developed  and  near 
the  ground.  The  group  is  hardly  to  be 
distinguished  as  a  family  from  the  Gall- 
inae.  It  contains  the  doves  and  pigeons 
(Coliunba,  Gonra,  Didunculus),  as  typi- 
cal members,  while  the  mound-birds 
(Megapodius),  the  curassows  (Crax),  and 
the  sand-grouse  (Pterocles')^  are  more 
aberrant.  The  dodo  (Did?ts),  extermi- 
nated about  two  centuries  ago,  was  an 
aberrant  pigeon.  The  humming-birds 

form  the  family  TROCHILID^E,  which  has  relations  with  the  picarian  birds. 

The  toes,  however,  are  three,  directed  forwards  as  in  the  preceding  groups. 


FIG.  342.  Wilson's  snipe, 
Gallinago  wi/som,  from  Tenney, 
after  Wilson. 


FIG.  343.     Bird  of  Paradise  {Paradisea  apoda),  female,  from  Hertwig, 
after  Levaillant. 


Other  characters  are  the  presence  of  basipterygoid  processes  and  the 
existence  of  a  single  carotid.  In  the  PICARI^E  the  first  and  fourth  toes 
are  directed  backwards,  while  the  palate  is  of  the  saurognathous  type 


BIRDS. 


351 


(p.  335)  .     Here  belong  the  woodpeckers  (Picus,  Colaptes)  and  the  wrynecks 


SECTION  III.  yEGITOGNATELE.  Birds  with  the  maxillopalatines 
not  united,  the  vomer  single,  broad,  and  notched  in  front.  In  the  family 
PASSERES,  which  embraces  over  half  the  known  species  of  birds,  three  toes 
are  directed  forwards.  These  birds  group  themselves  in  five  divisions,, 


FIG.  344.     Bird  of  Paradise  {Paradisea  apoda},  male,  from  Hertwig, 
after  Levaillant. 


typified  by  the  following  forms  :  The  lyre-bird  {Menurd),  the  broad-bills 
(Eurylamia})  the  tyrant-birds  and  king-birds  (Tyrannus),  the  ant-shrikes 
{Formic aria),  and  the  sparrows  {Passer},  those  allied  to  the  last  being  fre- 
quently known  as  Oscines  or  singing-birds.  Of  these  the  number  is  legion  \ 
no  attempt  can  be  made  here  to  even  enumerate  their  names. 


352  CLASSIFICATION  OF   VERTEBRATES. 

The  Euornithes  date  back  no  farther  than  the  eocene,  in  which  period 
representatives  of  the  cormorants,  pelicans,  flamingoes,  falcons,  kingfishers, 
•and  pheasants  appear.  The  colics,  oilbirds,  and  opisthocomi  are  not  known 
.as  fossils,  while  the  other  groups  appear  with  the  miocene. 

CLASS  II.     MAMMALIA. 

Hair-bearing  amniotes  with  two  occipital  condyles  ;  lower 
jaw  suspended  directly  from  the  cranium  without  the  interven- 
tion of  the  quadrate  ;  ankle  joint  between  the  tibia  and  fibula 
and  the  first  row  of  tarsal  bones ;  brain  with  well-developed 
corpus  callosum  ;  a  complete  diaphragm  ;  heart  four-chambered  ; 
only  one  (left)  aortic  arch  persisting ;  red  blood  corpuscles 
non-nucleate,  usually  circular  in  outline ;  eggs  (except  in 
monotremes)  minute,  and  undergoing  a  total  segmentation, 
the  embryonic  development  taking  place  inside  the  mother  ;  the 
young,  when  born,  nourished  by  milk  secreted  by  the  mammary 
.glands  of  the  mother. 

The  skin  in  the  mammals  has  a  well-developed  stratum 
-corneum  (p.  88),  which  is  never  molted  as  a  whole,  as  in  the 
reptiles  and  lower  vertebrates,  but  comes  away  piecemeal. 
The  skin  is  as  a  rule  pigmented,  and  the  pigment  may  occur 
in  either  the  deeper  (dermal),  or  more  superficial  (epidermal) 
portions.  The  epidermis  gives  rise  to  various  structures,  the 
most  noticeable  and  most  characteristic  of  which  is  hair,  the 
structure  and  development  of  which  is  described  elsewhere 
(P-  97)-  The  hair  is  usually  abundant,  and  covers  most  of  the 
body.  The  other  extreme  is  reached  in  the  cetacea,  where  it 
may  be  reduced  to  from  two  to  eight  pairs  of  bristles  in  the 
mouth  region,  these  occurring  in  some  cases  only  in  foetal  life. 
Frequently  one  can  distinguish  two  kinds  of  hair,  one  straight 
and  stiff,  covering  a  deeper  woolly  hair.  Hair  can  undergo  con- 
siderable modifications.  It  may  be  straight  or  curly  ;  it  may 
develop  into  bristles,  or  even  into  strong  protective  spines,  as  in 
.the  porcupines,  etc.  Frequently  certain  hairs  about  the  mouth 
•  (vibrissae)  have  tactile  functions,  their  roots  being  enveloped  in 
a  rich  plexus  of  nerve  fibres.  In  some  cases  the  hair  seems  to 
persist  throughout  life  (tail  and  mane  of  horses),  but  usually  it 
falls  out  and  is  replaced  by  new  hair,  this  molting  occurring 


MAMMALS. 


353 


gradually,  or,  as  in  the  case  of  many  inhabitants  of  colder 
climates,  before  and  "after  the  winter  season.  In  the  case  of 
some  arctic  species  this  molting  is  accompanied  by  color  changes, 
the  winter  pelage  being  white. 

Frequently  the  epidermal  layers  of  the  skin  becomes  greatly 
thickened  and  cornified,  producing  callosities,  or  thickened,  hair- 
less surfaces  like  those  found  on  the  soles  of  the  feet.  Cornifi- 
cation  of  the  epidermis  also  results  in  the  formation  of  horn, 
such  as  that  found  in  the  cavicornia  (cattle,  etc.),  and  in  the 
rhinoceros,  as  well  as  nails,  claws,  and  hoofs. 

Some  of  the  relations  between  the  epidermis  of  the  ap- 
pendages and  the  claws  are  interesting.  In  the  human  finger 


FIG.  345.  Diagrams  of  the  relations  of  nails,  claws,  and  hoofs,  after  Boas. 
A,  in  man;  B,  in  unguiculates;  C,  in  ungulates,  ^unmodified  epidermis;  SU, 
subungual  epidermis  (Sohlenhorn);  U,  nail. 

(Fig.  345  A),  there  exists  beneath  the  nail  a  peculiarly  modified 
epidermis,  su,  the  subungual  portion,  while  the  usual  epidermis,  s, 
covers  the  ball  of  the  finger.  In  the  unguiculates  (B)  the 
subungual  portion  is  more  developed,  and  forms  the  lower  sur- 
face, su,  of  the  claw.  In  the  ungulates  (Q,  the  unguis  becomes 
much  wider,  and  is  rolled  into  a  hoof,  on  the  lower  surface  of 
which  is  still  to  be  recognized  the  subungual  epidermis,  forming, 
for  instance,  in  the  case  of  the  horse,  the  sole,  into  which  there 
projects  behind  the  frog  of  the  foot,  which,  in  reality,  corre- 
sponds to  the  ball  of  the  finger  in  man. 

Scales  are  not  frequent  in  the  mammals.     They  occur  upon 
the  tails  of  certain  rodents,  and  upon  the  feet  of  these  and  some 


354  CLASSIFICATION  OF   VERTEBRATES. 

other  forms  (insectivores,  marsupials,  etc.).  More  rarely  the 
whole  body  may  be  encased  in  them  as  in  the  pangolins.  Again, 
as  in  the  armadillos,  dermal  bones  are  developed  in  connection 
with  the  scales,  while  in  some  embryonic  cetaceans  similar 
features  are  seen. 

Glands  are  far  more  abundant  than  in  the  sauropsida,  and 
include,  besides  the  common  sebaceous  and  sweat  glands,  numer- 
ous modifications,  usually  in  the  line  of  scent  glands.  These 
are  mostly  modifications  of  sebaceous  glands,  and  in  many  car- 
nivores, rodents,  and  edentates,  are  most  abundant  in  the  anal 
or  inguinal  regions.  In  other  groups  they  may  have  widely 
diverse  positions  ;  in  the  occipital  region  (camel),  in  the  lachry- 
mal bone  (many  ruminants),  upon  the  face  (bats),  on  the  legs 
(swine),  in  the  temporal  region  (elephants),  etc.  Here,  too, 
belong  the  problematical  glands  connected  with  the  spur  on 
the  hind  legs  of  the  monotremes. 

The  mammary  or  milk  glands  of  the  mammals  are  also 
modified  dermal  glands,  those  of  the  monotremes  most  closely 
resembling  sweat  glands,  those  of  other  mammals  sebaceous 
glands.  In  their  development  a  marked  ridge,  the  'milk  line/ 
appears  along  the  side  of  the  body,  certain  portions  of  which 
become  developed  into  the  glands,  the  intervening  portions 
aborting.  Connected  with  the  glands  are  the  teats  or  nipples, 
which  are  of  two  kinds  ;  the  one  produced  by  a  protrusion  of 
that  part  of  the  surface  upon  which  the  lacteal  glands  open ;  or 
(ungulates)  by  a  similar  elevation  of  the  surrounding  surface, 
the  openings  of  the  ducts  remaining  at  the  bottom  of  the  tube 
thus  formed  (Fig.  97).  The  number  of  teats  varies  between 
one  and  eleven  pairs  (Centetes).  These  may  be  distributed 
along  the  length  of  the  trunk,  or  may  be  restricted  to  either 
thoracic  or  abdominal  region. 

Except  in  a  very  few  forms  (hares)  there  is  a  layer  of  fat 
(panniculus  adiposus)  between  the  skin  and  the  muscles.  Be- 
sides, there  is  usually  a  layer  of  skin  muscles  (panniculus  carno- 
sus,  p.  115).  This  is  distinct  from  the  isolated  smooth  muscles 
connected  with  the  hair  follicles. 

In  the  skeleton  there  never  occurs  that  pneumaticity  found 
in  birds  and  some  extinct  reptiles,  the  cavities  of  the  bones  being 


MAMMALS.  355 

filled  with  marrow.  All  of  the  bones  except  those  of  the  skull, 
the  elements  of  the  sternum,  and  some  of  those  of  the  carpus 
and  tarsus,  are  provided  with  epiphyses,  —  separate  portions 
which  unite  later  in  life  with  the  rest  of  the  bone. 

As  a  rule  five  regions  —  cervical,  thoracic,  lumbar,  sacral,  and 
caudal  —  are  differentiated  in  the  vertebral  column,  but  in  the 
cetacea,  where  the  sacrum  is  lacking,  the  line  cannot  be  drawn 
between  lumbars  and  caudals.  The  cervicals,  which  are  almost 
constantly  seven l  in  number,  are  free,  except  in  most  cetacea  and 
some  edentates,  where  they  are  greatly  flattened  and  fused.  In 
some  rare  cases  they  bear  movable  cervical  ribs,  but  usually 
these  are  firmly  fused  to  dia-  and  parapophyses,  leaving  the  ver- 
tebrarterial  canal  to  betray  their  true  nature.  Usually  the  faces 
of  the  centra  are  flat,  but  opisthoccele  vertebrae  are  common  in 
the  necks  of  ungulates. 

The  trunk  or  dorso-lumbar  vertebrae  usually  number  nineteen 
or  twenty,  and  as  a  rule,  increase  in  the  number  of  thoracics  is 
correlated  with  a  reduction  of  the  number  of  lumbar  vertebrae. 
The  extremes  in  the  region  are  found  in  the  armadillo,  which  has 
fourteen,  and  Hyrax,  which  has  thirty  dorso-lumbar  vertebrae. 
The  number  of  thoracic  vertebrae  is  usually  thirteen,  but  it  is 
lower  in  bats  and  armadillos,  and  reaches  eighteen  in  the  horse, 
nineteen  or  twenty  in  the  rhinoceros  and  elephant,  and  twenty- 
three  or  four  in  the  three-toed  sloth.  The  lumbars  vary  from 
two  in  the  monotremes,  manatee,  and  two-toed  ant-eaters,  to  nine 
in  Stenops,  the  usual  number  being  six  or  seven. 

The  sacral  vertebrae  are  primitively  two  in  number,  but 
others  taken  from  the  lumbar  and  caudal  regions  may  unite  by 
synostosis  with  the  ilium,  giving  a  total  number  of  sacral  verte- 
brae of  eight  or  nine  in  the  sloths  and  armadillos.  The  caudals 
are  extremely  variable  in  number  and  are  usually  numerous,  the 
number  being  greatly  reduced  only  in  the  anthropoid  apes  and 
man. 

The  ribs  (corresponding  in  number  to  the  thoracic  vertebrae) 
are  bicipital,  being  furnished  with  tubercular  and  capitular  heads, 
the  former  articulating  with  the  diapophysis  (the  transverse 

1  Manatus  australis  and  Choloepus  hoffmanni  have  six,  Bradypus  torquatus  has 
eight,  and  B.  tridactylus  nine  cervicals. 


356  CLASSIFICATION  OF  VERTEBRATES. 

process  of  human  anatomy)  the  latter  with  articular  facets  (re- 
duced parapophyses)  on  the  centra.  The  ribs  usually  present 
bony  and  cartilaginous  portions,  the  latter  reaching  the  sternum. 
In  floating  ribs  the  union  with  the  sternum  is  lacking,  while  false 
ribs  are  without  vertebral  connections. 

The  sternum  frequently  retains  throughout  life  the  separate 
elements  or  sternebrae  of  which  it  is  composed ;  but  these  may 
fuse  into  an  elongate  plate,  the  corpus  sterni  or  mesosternum, 
with  an  anterior  portion,  the  manubrium  or  presternum,  and  a 
posterior  xiphisternum  or  ensiform  process  with  which  no  ribs 
articulate.  The  episternum  (p.  149),  which  is  laid  down  in  carti- 
lage, is  placed  in  front  of  the  sternum,  but  retains  its  distinctness 
only  in  the  monotremes  and  some  edentates  and  rodents.  In 
other  forms  it  fuses  with  the  sternum. 

In  the  mammalian  skull  there  is  a  more  intimate  relationship 
between  the  cranial  and  facial  elements  than  is  the  case  in  the 
lower  vertebrates.  There  is  also  a  marked  tendency  to  the 
fusion  of  bones  which  are  distinct  in  the  lower  vertebrates,  but 
usually  the  process  of  co-ossification  is  not  complete  except  in  the 
hyoid  and  lower  jaw,  many  of  the  bones  being  suturally  united 
throughout  life.1  The  floor  of  the  skull  is  preformed  in  car- 
tilage, its  roof  of  membrane  bones.  No  interorbital  septum 
occurs,  its  remnants  being  found  in  the  crista  galli  process  of  the 
ethmoid,  while  the  lateral  walls  continue  forward  to  the  ethmoid 
region.  Basi-  and  presphenoids  frequently  fuse,  and  from  the 
sphenoid  thus  formed  a  greater  and  a  lesser  wing  arises  on  either 
side,  these  being  the  ali  and  orbitosphenoids  respectively  of  the 
lower  forms.  The  pterygoids  also  unite  with  the  sphenoids, 
forming  the  pterygoid  processes. 

Basi-,  ex-,  and  supraoccipitals  may  remain  distinct,  or  they 
may  fuse,  sometimes  late  in  life,  into  a  single  occipital  bone 
which  bears  a  pair  of  occipital  condyles  arising  from  the  exoccip- 
itals,  the  basioccipital  but  rarely  contributing  to  their  formation. 

The  sides  of  the  skull  are  formed  in  part  by  the  sphenoidal 
alae,  in  part  by  the  temporal  bones,  each  of  which  is  a  complex 
of  several  elements,  —  the  petrosal  (fused  pro-,  epi-,  and  opi- 

1  The  obliteration  of  .sutures  has  progressed  the  farthest  in  monotremes,  the  weasel 
and  some  apes. 


MAMMALS. 


357 


sthotics)  squamosal,  mastoid,  and  tympanic.  These  are  frequently 
fused.  The  tympanic,  surrounding  the  external  auditory  mea- 
tus  may  develop  into  a  saccular  auditory  bulla.  In  front  the 
temporal  gives  off  a  zygomatic  process  which  extends  forward 
to  join  the  jugal,  or,  as  it  is  called,  the  malar  bone. 

An  interparietal  may  be  distinct,  as  in  rodents,  or  it  may 
fuse  with  the  supraoccipital,  or,  more  rarely,  with  the  parietals. 
The  frontals  are  usually  paired,  and  in  the  ungulates  they  may 


e.n 


FIG.  346.  Skull  of  young  Tatusia,  from  Wiedersheim  ;  cartilage  dotted. 
aty,  tympanic  annulus;  bhy,  basihyal;  chy,  ceratohyal;  cr,  cricoid;  d,  dentary;  ehy, 
epihyal;  en,  external  nares;  eo,  exoccipital;  f,  frontal;  hhy,  hypohyal;  /',  jugal; 
in,  incus;  Ic,  lachrymal;  mk,  Meckel's  cartilage;  ml,  malleus;  mx,  maxillary;  ;/, 
nasal;  occ,  occipital  condyle;  /,  parietal;  pa,  palatine;/.*-,  premaxilla;  sq,  squa- 
mous  part  of  temporal;  st,  stapes;  stm,  stapedial  muscle;  so,  supraoccipital;  th, 
thyroid  cartilage;  tr,  trachea;  II,  V,  passages  of  nerves. 

develop  bony  horns.  Frequently  each  gives  off  a  postorbital 
process  which,  approaching  or  meeting  the  jugal,  partially  or 
completely  separates  the  orbit  from  the  temporal  fossa.  Post- 
frontals  are  lacking. 

The  cranium  is  closed  in  front  by  the  ethmoid  bone  in  which 
may  be  recognized  a  median  (mesethmoid)  plate  which  divides 
the  nasal  cavity  into  right  and  left  halves,  and  on  either  side  a 
lamina  cribrosa  perforated  for  the  passage  of  the  olfactory  nerve. 


358  CLASSIFICATION  OF   VERTEBRATES. 

The  cribrosa,  together  with  the  superior  turbinal  bones  in  the 
nasal  passage,  apparently  represents  the  prefrontal  of  lower 
vertebrates. 

The  nasal  cavity  is  bounded  externally  by  the  nasal  bones, 
which  are  small  in  cetacea  and  fused  in  the  old-world  apes. 
Inside  the  cavity,  besides  the  superior  turbinals  already  referred 
to,  are  the  inferior  turbinals  which,  beginning  as  separate  ossi- 
fications, fuse  with  the  maxillaries.  The  septum  of  the  nose, 
established  by  the  mesethmoid,  is  continued  by  the  vomer,  in 
which  the  paired  bones  of  the  lower  forms  are  fused,  and  are 
entirely  cut  off  from  the  roof  of  the  mouth. 

Among  the  most  characteristic  features  of  the  facial  skele- 
ton are  the  close  union  of  the  maxillopalatine  region  with  the 
rest  of  the  skull,  and  the  suspension  of  the  lower  jaw  direct  from 
the  temporal  bone  without  the  intervention  of  a  quadrate.  The 
premaxillae  may  fuse  with  the  maxillae,  while  maxillae  and  pala- 
tines send  off  horizontal  palatine  processes,  which,  meeting  in 
the  middle  line,  form  the  hard  palate,  bound  the  nasal  cavities 
below,  and  carry  the  choana  far  back.  The  pterygoids  may  also 
contribute  to  the  hard  palate  (some  edentates). 

The  lower  jaw  consists  of  but  a  single  bone,  the  dentary,  on 
either  side,  or  the  two  halves  may  anchylose  at  the  symphysis. 
In  the  middle  ear  are  three  small  bones  which  form  a  sound- 
conducting  apparatus  leading  from  the  tympanic  membrane  to 
the  fenestra  ovalis.  In  order  from  outside  in  these  are  the  mal- 
leus, incus,  and  stapes.  Concerning  their  homologies  the  most 
diverse  views  are  held,  both  incus  and  malleus  having  been 
regarded  as  the  missing  quadrate.  The  views  of  the  homologies 
given  on  p.  I  59  seem  to  be  in  full  accord  with  the  results  of 
the  studies  of  most  students  who  have  approached  the  subject 
from  the  embryological  standpoint. 

In  the  mammals  the  brain  exercises  great  influence  upon 
the  shape  of  the  skull.  As  it  nearly  fills  the  cranial  cavity, 
increase  in  its  size  can  only  be  accommodated  by  an  outgrowth 
of  the  cranial  walls.  To  measure  the  extent  of  this  outgrowth 
and  thus  approximately  to  obtain  an  index  of  cerebral  develop- 
ment, the  facial  angle  is  employed.  According  to  the  system 
of  Camper  this  is  the  angle  formed  by  two  lines,  one  passing 


MAMMALS. 


359 


from  the  auditory  meatus  to  the  base  of  the  nose,  the  other 
from  the  base  of  the"  nose  to  the  most  prominent  part  of  the 
frontal  bone.  Less  used  is  the  system  in  which  the  lines  inter- 
sect at  the  insertion  of  the  teeth  in  the  upper  jaw.  In  man 
Camper's  angle  varies  from  70°  to  nearly  90°.  In  monkeys 
from  60°  (Chrysothrix)  to  35°  or  30°  ;  in  other  mammals  it  is  25° 
or  lower.  v 

The  hyoid  arch  is  connected  dorsally  with  the  otic  capsule, 
ventrally  with  the  first  branchial  (Fig.  31).  In  it  ossifications 
take  place  which  proceed  to  varying  extents.  The  whole  arch 
may  ossify,  giving  the  separate  elements,  basi-,  hypo-,  cerato-, 
epi-,  and  stylohyal,  or  the  median  portion  on  either  side  may  be 
converted  into  a  stylohyal  ligament,  the  stylohyal  element  fus- 
ing with  the  skull  and  forming  the  styloid  process,  while  the 
basi-  and  epihyals  fuse  to  form  the  body  and  lesser  horn  of  a 
single  hyoid  bone.  The  first  branchial  arch  also  fuses  with 
this,  contributing  to  the  body,  and  forming  a  greater  horn  on 
either  side.  There  is  also  a  ligamentary  connection  with  the 
second  and  third  branchial  arches  (thyroid  cartilage,  p.  28). 

The  pectoral  girdle  shows  many  variations  from  the  typical 
condition,  for  while  the  scapula  is  always  present,  the  coracoid, 


A  B 

FIG.    347.     Pelvis  (A^)  of  young  Ornithorhynchus ;  (ff~)  of  calf,  after  Boas. 
A,  acetabulum;    IL,  ilium ;    IS,  ischium;    M,  marsupial  bone;    P,  pubis. 

except  in  the  monotremes,  is  reduced  to  a  small  element  —  the 
coracoid  process  —  fused  to  the  scapula.  The  presence  or  ab- 
sence of  a  clavicle  is  correlated  with  habits  ;  flying,  digging,  and 
climbing  mammals  having  it,  while  it  is  absent  in  whales,  ungu- 
lates, and  many  others.  In  rodents  and  carnivores  it  is  reduced, 
and  has  only  ligamentary  connections.  The  pelvis,  on  the  other 


360  CLASSIFICATION  OF   VERTEBRATES. 

hand,  is  more  normal,  except  in  the  cetacea,  where  it  is  apparently 
absent,  the  one  or  two  rib-like  bones  which  occur  imbedded  in 
the  muscles  free  from  the  vertebral  column  being  usually  inter- 
preted as  femur  and  tibia.  In  all  others  the  ilia  are  united  to 
the  sacral  vertebrae,  while  with  rare  exceptions  the  pubes  of  the 
two  sides,  and  usually  the  ischia  as  well,  unite  in  a  symphysis 
below.  In  monotremes  and  marsupials,  marsupial  bones,  prob- 
ably epipubic  in  character  (p.  171),  are  developed  from  the 
cephalic  side  of  the  pelvic  girdle. 

In  the  skeleton  of  the  free  appendages  great  variations  occur, 
especially  in  the  direction  of  reduction  of  digits,  etc.;  and  for 
details  of  these  reference  must  be  made  to  the  accounts  of  the 
separate  orders.  In  general  it  may  be  said  that  the  tendency 
is  towards  a  reduction  in  the  number  of  digits,  and  towards  an 
alternation  and  interlocking  of  the  carpal  and  tarsal  bones.  In 
the  swimming-forms  there  is  also  a  shortening  of  the  limbs, 
the  reduction  going  so  far  that  in  the  case  of  the  sirenia  only  the 
elbow  joint  is  functional,  while  in  the  whales  even  this  joint  is 
lost.  On  the  other  hand,  in  these  forms,  as  in  the  ichthyosaurs 
and  plesiosaurs,  an  increase  in  the  number  of  phalanges  is  more 
or  less  marked.  In  the  bats,  on  the  other  hand,  the  elongation 
of  most  of  the  digits  of  the  hand,  and  their  utilization  as  sup- 
ports of  the  wing,  is  noticeable. 

In  detail :  the  humerus  may  be  either  long  or  short,  the  con- 
ditions here  being  usually  in  reversed  correlation  to  those  found 
in  the  metacarpus.  The  ulna  and  radius  are  usually  longer 
than  the  humerus,  the  ulna  being  produced  beyond  the  hinge  of 
the  elbow  as  an  olecranon  process.  The  radius  is  more  closely 
related  to  the  carpus,  and  is  capable  of  turning  more  or  less 
freely  around  the  ulna  in  the  process  of  pronation  and  supination 
of  the  manus.  Occasionally  radius  and  ulna  coalesce. 

The  femur  usually  bears  two  or  three  enlargements  (tro- 
chanters)  for  the  attachment  of  muscles.  At  the  knee  joint  a 
patella  or  knee-pan  usually  occurs.  The  tibia  and  fibula  are 
usually  longer  than  the  femur,  and  in  the  marsupials  are  capable 
of  marked  rotation.  On  the  other  hand,  the  fibula  is  frequently 
reduced,  and  united  more  or  less  closely  to  the  tibia. 

The  greatest  variations  occur  in  the  carpus  and  tarsus  and 


MAMMALS. 


36  L 


more  distal  portions.  The  carpal  and  tarsal  bones  are  in  twcr 
or  three  rows,  those  "of  the  distal  row  being  opposite  or  alter- 
nate with  the  others.  In  the  tarsus  the  os  calcis  and  astragalus 
are  the  most  prominent,  the  former  being  the  fibulare,  the  latter 
fused  tibiale  and  intermedium  (p.  176).  The  digits  are  typically 
five  in  number,  but  Pedetes  presents  structures  usually  inter- 
preted as  a  sixth  toe.  The  tendency  is  constantly  towards  re- 
duction in  the  number  of  digits,  disappearance  being  preceded 
by  a  reduction  in  length,  in  which 
case  the  metacarpals  are  shortened 
and  are  occasionally  reduced  to  splint 
bones.  In  certain  groups  there  fre- 
quently occurs  a  fusion  of  the  two 
middle  metacarpals.  The  phalanges 
in  the  digits  never  exceed  three,  ex- 
cept in  the  whales.  Mammals  are 
spoken  of  as  plantigrade,  digitigrade, 
or  unguligrade,  accordingly  as  they 
walk  upon  the  whole  metacarpal  or 

metatarsal  region,  as  in  the  bear  and 

'        ....    .       ,     ,  FIG.  348.    Fore  (right)  and 

man  ;   or  upon   the   distal  phalanges,      hind  (left)  feet  of  tapir.     «,. 

as    in    the    Cats    and    dogs  ;     Or,   again,        astragalus;    c,     cuneiforme    in 

.   calcaneum  in    hind. 


fore 

foot ;   c",  c'" ,  cuneiforme ;  cbY 

cuboid  ;  7%  femur  ;    /,  lunare  ; 


upon    the    nails    (hoofs),    as    in    the 
horse  and  cow. 

The    most    Striking  feature  of  the        m,    magnum;    «,    naviculare;: 

nervous  system  of  existing  mammals      *>  pisiforme;   A\  radius;   *„ 

......  scaphoid ;     /,     trapezoid ;      7'., 

is  the  great  size  of  the  brain,  and  es-      tibia.  tt>  unciforme;  u,  ulna, 
pecially   of   the    cerebrum    and   cere- 
bellum, the  former  overarching  twixt  and  mid  brains  and  reach- 
ing the  latter.      In  the  lower  mammals  the  cerebral  surface  is 
smooth,  but  in  the  higher  it  is  marked  by  gyri   and  convolu- 
tions, the  effect   of   which  is  to  increase   the  amount   of   sur- 
face  and   consequently  of   gray  matter.1     This   great   increase 
of  the  cerebrum  is   largely   an   increase  of   the   pallium,   only 
a   small   portion  of  which  remains  epithelial  in  character.      In 


1  By  some  authors  the  mammals  are  sub-divided  into  two  groups,  —  Ineducabilia,  With, 
smooth  cerebra  (Fig.  349),  and  Educabilia.  in  which  the  surface  of  the  cerebrum  is  convo- 
luted (Fig.  54). 


362 


CLASSIFICATION  OF   VERTEBRATES, 


the  aplacental  mammals  the  anterior  commissure  is  especially 
well  developed,  and  forms  the  chief  connection  between  the 
two  sides  of  the  brain,  while  the  corpus  callosum  remains  more 
rudimentary,  as  in  sauropsida.  In  the  placentalia,  on  the  other 
hand,  the  corpus  callosum  or  commissure  between  the  two 
hemispheres  becomes  the  most  important  connection  between 
the  right  and  left  sides,  the  anterior  commissure  remaining 

behind.  The  longitudinal 
commissures,  the  fornix,  and 
the  cornua  ammonii,  are  also 
well  developed.  The  lateral 
ventricles  are  large,  and  in 
them  several  sub-regions  are 
distinguished, —  anterior  and 
descending,  and  in  the 
higher  mammals  posterior 
cornua.  The  olfactory  lobes 
are  comparatively  small,  and 
in  the  whales  are  absent. 
In  development  a  diverticu- 
ff  lum  of  the  lateral  ventricle 
extends  into  each  olfactory 
lobe,  but  except  in  a  few 
forms,  like  the  horse,  this 
disappears  in  the  adult. 

The  tvvixt  brain  and  op- 
tic lobes  (corpora  bi-  or 
quadrigemini)  are  poorly 

developed,    and    are    covered 

in    by  the    hinder    lobe    of 

the  cerebrum.  The  epiphysis  is  small  and  lacks  any  sensory 
structures.  In  the  cerebellum  the  vermis  is  large  in  the  apla- 
centals,  but  in  the  placentalia  the  lateral  lobes  of  the  cere- 
bellum are  in  the  ascendancy.  A  corresponding  increase  from 
lower  to  higher  is  seen  in  the  pons  varolii.  The  spinal  cord 
extends  back  only  to  the  sacral  region,  the  posterior  part  of 
the  spinal  canal  being  occupied  by  a  cauda  equina  formed  of 
the  more  posterior  nerves  before  their  exit. 


JUI 
Wu 
NF1 

FIG.  349.  Brain  of  rabbit  from  above, 
'from  Wiedersheim.  Bol,  olfactory  bulb; 
Fip,  pallial  fissure;  Gp,  pinealis;  ////,  cere- 
bellar  hemispheres ;  A"//,  medulla ;  VH, 
^cerebral  hemispheres;  Wu>  vermis. 


MAMMALS.  363 

The  olfactory  organ  is  noticeable  for  the  great  increase  of 
the  olfactory  epithelium  and  the  corresponding  complexity  of 
the  bony  labyrinth  which  supports  it.  In  the  formation  of  this 
labyrinth  small  bony  processes  play  the  greater  part,  the  upper 
(superior  turbinal)  being  an  outgrowth  from  the  ethmoid,  the 
lower  (inferior  turbinal)  usually  uniting  with  the  maxillary. 
The  two  cavities  are  separated  behind  by  the  ethmoid  and 
vomer,  the  partition  being  continued  to  the  tip  of  the  nose  by 
cartilage.  Connected  with  the  nasal  cavities  are  numerous 
sinuses  in  the  frontal,  maxillary,  and  sphenoid  bones.  The  ol- 
factory nerve  breaks  up  into  numerous  fibres  before  leaving  the 
cranial  cavity,  and  these  pass  through  the  perforations  in  the 
cribiform  plate  of  the  ethmoid. 

The  eyes  vary  according  to  the  habits,  being  small  in  bur- 
rowing forms,  or  even  occasionally  without  muscles  and  beneath 
the  skin  {Spalax,  Chrysochloris).  As  a  rule  the  eyeball  is  ap- 
proximately spherical,  except  in  the  whales,  where  it  is  flattened. 
It  is  placed  in  orbits,  usually  incomplete,  and  these  are  more  or 
less  lateral  in  position.  Sclerotic  bones  are  never  developed. 
Besides  the  upper  and  lower  lids,  a  nictitating  membrane  is 
usually  well  developed,  but  sometimes,  as  in  man,  this  is  reduced 
to  a  small  muscleless  fold,  the  plica  semilunaris,  at  the  inner 
angle  of  the  eye.  In  the  sirenia  the  eyelids  act  as  an  iris-like 
diaphragm.  Frequently  a  seventh  muscle  of  the  eye,  a  re- 
tractor bulbi,  is  present,  and  in  the  carnivores  this  is  four-divided. 
In  carnivores,  dolphins,  ungulates,  and  some  marsupials,  a  metal- 
lic lustre  is  developed  in  a  part  of  the  choroid  (tapetum). 

The  ears  are  marked  externally,  except  in  monotremes,  ce- 
tacea,  and  some  seals,  by  the  development  of  a  conch,  supported 
by  cartilage,  and  moved  by  appropriate  muscles.  From  this  the 
external  meat  us  leads  inward  to  the  tympanum,  which  is  crossed 
by  the  three  ossicula  auditus  —  malleus,  incus,  and  stapes  —  al- 
ready mentioned  (p.  358).  From  the  tympanum  the  Eustachian 
tube  (p.  73)  leads  to  the  pharynx,  except  in  the  whales,  where 
it  enters  the  nasal  passages.  The  inner  ear  is  characterized 
by  the  great  development  of  the  lagena  which  coils  with  two 
or  three  turns  to  form  the  spiral  cochlea.  The  monotremes 
have  the  inner  ear  more  on  the  sauropsidan  plan. 


364  CLASSIFICATION  OF   VERTEBRATES. 

Except  in  the  monotremes,  which  are  provided  with  a  horny 
beak,  the  mouth  of  the  mammals  is  bounded  by  movable  lips 
near  the  margins  of  the  jaws ;  between  the  lips  and  the  jaws  on 
either  side  are  the  cheeks,  and  in  many  apes  and  rodents  these 
are  developed  into  large  cheek-pouches,  sometimes  hairy  on  the 
inner  surface.  A  tongue  is  always  present.  In  the  cetacea  it 
is  immovable,  in  all  others  it  is  mobile  and  sensory,  and  in  the 
giraffe  it  is  even  prehensile.  At  its  base  it  bears  the  papillae 
circumvallatas,  special  aggregations  of  taste  buds.  Beneath 
the  tongue  is  a  single  or  double  under-tongue  or  sublingua, 
especially  developed  in  the  insectivores,  which  may  be  the  ho- 
mologue  of  the  tongue  of  the  lower  vertebrates,  the  functional 
tongue  being  a  new  formation.  Three  pairs  of  salivary  glands 
are  present  (parotid,  submaxillary,  and  sublingual)  except  in  the 
carnivorous  cetacea.  The  secretion  is  most  abundant  in  the  her- 
bivorous mammals.  A  soft  palate  is  alway  present,  behind 
which  are  the  choanae. 

The  teeth  show  a  greater  range  of  variation  than  is  found  in 
any  other  group  of  vertebrates.  They  are  lacking  in  but  few 
forms,  as  Echidna,  Mams,  and  the  baleen  whales,  while  in  the 
adult  Ornithorhynchus  they  are  replaced  by  cornified  teeth,  al- 
though true  teeth  are  present  in  the  young.  They  never  have 
such  an  extensive  distribution  as  is  found  in  reptilia  and  ichthy- 
opsida,  but  are  confined  to  the  margins  of  maxillary,  premaxil- 
lary,  and  mandibular  (dentary)  bones.  In  all  cases  they  are 
thecodont,  i.e.,  are  situated  in  sockets  or  alveoli,  although,  as  in 
the  dolphins,  the  sockets  may  run  together  into  a  continuous 
groove,  while  in  some  shrews  the  molars  in  the  adult  are  firmly 
anchylosed  to  the  jaws. 

In  a  few  cases,  as  in  the  denticete  whales,  the  teeth  are  all 
similar  in  form  (homodont),  but  usually  they  are  differentiated 
(heterodont)  into  incisors,  canines,  and  molars.  The  incisors 
are  placed  in  the  premaxilla,1  and  in  a  corresponding  position  in 
the  lower  jaw.  The  canines  are  placed  behind  the  suture  sepa- 
rating maxillary  and  premaxillary  bones,  and  never  exceed  one 
on  a  side  in  either  jaw.  Behind  the  canines  are  the  molars.  In- 
cisors and  canines  have  single  roots,  and  the  crown  is  usually  a 

1  In  some  armadillos  the  premaxillary  teeth  cannot  be  incisors. 


MAMMALS.  365 

simple  cone  or  chisel.  The  molars  vary  greatly  in  shape  and 
structure,  and  may  have  several  roots,  a  feature  not  found  in 
other  living  vertebrates. 

The  shapes  and  modifications  of  the  molars  are  of  great 
value  in  classification,  and  a  few  definitions  may  prove  of  use 
in  reading  the  descriptions  of  systematic  works.  In  the  more 
primitive  teeth,  each  tooth,  no  matter  where  placed,  h'as  the  shape 
of  a  simple  cone,  as  in  the  denticete  whales  (haplodont),  but  usu- 
ally the  crowns  of  the  molars  present  crests,  prominences,  tuber- 
cles, etc.  There  are  two  views  as  to  the  origin  of  the  more 
complex  condition.  According  to  the  one,  the  typical  mam- 
malian molars  have  arisen  by  the  fusion  of  several  simple  teeth, 
like  those  of  many  reptiles.  The  other  view  is  that  accessory 
prominences  have  been  developed  upon  the  primary  tooth,  a  view 
which  has  much  in  its  favor.  First  to  appear 
of  these  more  complex  teeth  was  the  tricono- 
dont,  in  which  secondary  prominences  —  cones 
in  the  upper  jaw,  conids  in  the  lower  —  were 
developed  in  a  straight  line,  a  paracone  (para-  FIG.  350. 
conid)  in  front  of  the  primary  or  protocone,  and  Bunodont  tooth 
a  metacone  behind.  In  the  tritubercular  tooth,  baboon) 
which  came  next  in  turn,  the  three  cones  are 
arranged  in  a  triangle,  the  protocone  on  the  inner  side,  the  pro- 
toconid  on  the  outer.  This  part  of  the  tooth  forms  the  trigon, 
and  from  this  modifications  may  be  developed  in  different  direc- 
tions. Thus,  while  retaining  its  tubercular  character,  a  posterior 
lower  heel  or  talon  may  be  formed,  and  when  this  develops  a 
single  tubercle  it  is  known  as  the  hypocone  or  hypoconid,  the 
former  occurring  at  the  postero-internal  angle  of  the  upper 
molars,  the  latter  at  the  postero-external  angle  of  the  lower. 
Protoconule  and  metaconule  are  smaller  intermediate  cusps, 
while  the  crests  which  connect  the  cones  and  conids  are  known 
as  lophs.  Again,  peripheral  cusps  or  styles  may  arise  outside 
these  from  the  girdle  or  cingulum  of  the  tooth. 

The  tubercles  and  lophs  of  the  teeth  also  vary  in  character. 
When  the  surface  is  calculated  for  cutting,  the  tooth  is  secodont, 
when  for  crushing,  it  isbunodont;  with  the  development  of  prom- 
inent transverse  crests  the  tooth  becomes  lophodont.  When  the 


366  CLASSIFICATION  OF   VERTEBRATES. 

crests,  crescentic  in  character,  have  a  longitudinal  direction,  the 
tooth  is  selenodont.  These  characteristics  may  be  combined, 
giving  the  types  called  bunolophodont,  bunoselenodont,  etc. 

The  teeth  first  formed  may  be  the  only  ones  to  appear 
during  life,  when  we  speak  of  a  monophyodont  condition  as  in 
the  monotremes,  cetacea,  and  edentates  ;  or  again,  we  may  have 
a  first  or  milk  dentition  to  be  replaced  later  by  a  permanent  set 
(diphyodont1).  Milk  and  permanent  dentitions  are  not  exact 
repetitions  of  each  other,  more  molars  appearing  in  the  perma- 
nent than  in  the  milk  dentition.  This  leads  to  a  differentiation 
of  the  molars  into  premolars  (bicuspids  of  the  dentist)  which 
occur  in  both  sets,  and  molars  proper,  which  appear  only  with 
the  permanent  dentition. 

In  homodont  dentition  the  number  of  teeth  is  very  large, 
and  may  vary  between  one  and  two  hundred.  In  heterodont 
types  this  number  is  greatly  reduced.  It  is  greatest  in  the 
marsupials,  where  there  may  be  five  incisors  and  six  molars  on 
either  side  in  either  jaw.  In  placenta!  mammals  the  incisors 
never  exceed  three,  and  the  full  dentition  may  be  stated  as 
including  44  teeth.  JMot  infrequently,  as  in  rodents  and  rumi- 
nants, the  canines  may  be  "lacking,  producing  a  gap  or  diastema 
between  incisors  and  premolars,  while  not  infrequently  the 
incisors  may  not  be  developed  in  the  adult. 

To  express  in  concise  form  the  number  of  teeth  present  in 
any  mammal  —  a  matter  of  great  importance  in  classification  — 
a  dental  formula  has  been  adopted,  in  which  the  kinds  of  teeth 
are  represented  by  the  letters  z,  c,  p,  and  m,  while  the  number 
of  teeth  in  upper  and  lower  jaws  are  represented  by  figures 
above  and  below  a  line.  Since  the  two  halves  of  either  jaw  are 
mirror-like  repetitions,  only  the  teeth  in  one  side  are  repre- 
sented. The  dental  formula  of  the  adult  man  is  expressed 
thus:- 

i  f,  c  },/  |,  m  I  =  32; 
that  of  the  horse,  i  §,  c  \,  p  •},  m  %  =  40  ; 
that  of  the  cow,  i  §,  c  %,  p  |,  m  I  =  30. 

1  Stirling  has  described  teeth  in  the  marsupial  Myrmecobius,  formed  before  the  milk 
set,  which,  taken  in  connection  with  the  studies  of  Kuchenthal  and  Rose,  show  that  the 
marsupials,  like  most  other  mammals,  are  diphyodont,  and  may  lead  to  the  conclusion  that 
the  milk  dentition  must  be  a  second  set,  and  the  permanent  teeth  a  third. 


MAMMALS. 


367 


A  few  other  facts  concerning  the  teeth  may  be  added. 
Occasionally,  as  in  rodents  and  elephants,  the  incisors  may 
have  persistent  pulps,  and  hence  may  continue  to  grow  through- 
out life.  The  enamel  is  lacking  in  a  few  forms,  like  the  eden- 
tates and  the  dugongs.  The  milk  dentition  is  lacking  in  some 
rodents  ;  in  the  guinea-pig  the  milk  dentition  is  shed  in  utero, 
and  in  the  seal  it  never  cuts  the  gums.  Finally,  there  is  such 
correlation  between  the  teeth  and  other  structural  features,  that 
the  dentition  affords  an  index  to  the  classification,  and  hence 
becomes  of  great  assistance  to  the  paleontologist. 


FIG.  351.  Diagrams  of  stomachs  of,  A,  horse;  B,  pig;  C,  Lagenorhynchus  ; 
D,  ziphioid  whale;  E,  seal;  F,  rat.  d,  duodenum;  o,  oesophagus;  /,  pylorus; 
oesophageal  region  horizontally  lined ;  cardiac  gland  region  obliquely  lined ;  fundus 
gland  region  dotted;  pyloric  gland  region  with  crosses;  after  Oppel. 

The  oesophagus  is  greatly  elongated,  extending  from  the 
pharynx  through  the  diaphragm  to  the  stomach.  Usually  the 
stomach  is  regarded  as  the  saccular  enlargement  of  the  alimen- 
tary canal,  lying  between  the  oesophagus  and  the  intestine ;  but 
when  histological  and  physiological  features  are  taken  into 
account,  it  is  seen  that  frequently  the  lower  end  of  the  oesoph- 
agus expands,  and  takes  part  in  the  formation  of  the  gastric 
enlargement,  and  that  the  stomach  proper  begins  only  where 
the  gastric  glands  appear.  Of  these  glands  three  kinds  are 
recognized,  cardiac,  fundus,  and  pyloric,  for  the  characters  of 


368  CLASSIFICATION  OF   VERTEBRATES. 

which  reference  must  be  made  to  histological  text -books.  On  the 
basis  of  these  glands  the  stomach  may  be  divided  into  regions, 
when  it  is  seen  that  in  the  monotremes  the  morphological  stom- 
ach is  entirely  lacking,  the  enlargement  which  occurs  being 
resophageal.  More  frequently  the  cardiac  glands  are  lacking. 

Speaking  of  the  stomach  in  the  more  usual  sense,  it  may  be 
said  that  usually  its  axis  lies  at  right  angles  to  the  axis  of  the 
body,  and  that  only  exceptionally,  as  in  the  seals,  is  it  longitu- 
dinal.1 The  stomach  may  be  a  simple  sac,  as  in  man,  but  in 
the  cetacea  and  ruminants  it  becomes  divided  into  several  cham- 
bers, the  extremes  of  differentiation  being  reached  in  the  rumi- 
nants, where  (the  tylopoda  and  tragulina  excepted)  four  distinct 
-  regions,  the  rumen  (paunch),  reticulum  (honeycomb),  psalte- 
rium,  or  omasum  (manyplies),  and  abomasum  may  be  recog- 
nized ;  but  of  these  only  the  last  is  a  true  glandular  stomach, 
the  others  being  oesophageal  enlargements. 

The  intestine  is  differentiated  into  small  and  large  divisions, 
the  line  between  them  being  marked  by  the  ileocolic  valve. 
The  first  part  of  the  intestine,  the  duodenum,  is  characterized 
by  receiving  the  ducts  of  liver  and  pancreas,  and  also  by  the 
presence  of  Brunner's  glands  in  its  walls.  The  small  intestine 
is  greatly  convoluted.  The  large  intestine  is  of  larger  diameter 
than  the  small,  and  its  walls  show  outsackings  or  lobulations. 
It  presents  two  divisions,  a  rectum,  situated  in  the  pelvis,  and 
comparable  to  the  large  intestine  of  the  lower  vertebrates,  and 
a  much  longer  colon,  which  appears  for  the  first  time  in  mam- 
mals. Just  beyond  the  ileocolic  valve  is  a  blind  diverticulum, 
the  caecum,  which  undergoes  great  variations  in  size.  It  is 
largest  in  the  herbivora,  where  it  may  equal  the  body  in  length, 
while  in  the  edentates,  carnivores,  toothed  whales,  bats,  etc.,  it 
may  be  small  or  even  absent.  In  many  rodents,  apes,  and  man, 
the  distal  part  of  the  caecum  becomes  reduced  in  size,  and  forms 
the  appendix  vermiformis.  The  rectum  terminates,  except  in  the 
monotremes,  in  an  anus  dorsal  to  the  urogenital  opening.  In 
that  group  it  and  the  urogenital  system  empty  into  a  cloaca  as 
in  the  sauropsida. 

1  In  the  seals  (Fig.  351  £),  it  is  only  the  oesophageal  part  of  the  stomach  that  is  longi- 
tudinal, the  true  stomach  being  transverse. 


MAMMALS. 


369 


The  liver,  often  lobed  in  a  complicated  manner,  is  divided 
into  right  and  left  halves  by  the  ligamentum  teres,  itself  a  ves- 
tige of  the  earlier  umbilical  vein.  The  left  half  is  frequently 
sub-divided  into  left  and  central  lobes,  while  the  right  also  is 
usually  sub-divided,  and  may  have  a  caudate  lobe  laterally  placed, 
while  a  spigelian  lobe  projects  dorsal  to  the  entrance  of  the 
portal  vein.  A  gall  bladder,  which  arises  as  a  diverticulum 
of  the  hepatic  duct,  is  rarely  absent  (horse,  whales,  and  some 
rodents).  The  pancreas  is  usually  compact,  but  in  some  rodents 
it  is  diffuse.  Its  duct,  as  a  rule,  unites  with  the  hepatic  duct ; 
but  occasionally  these  may 
empty  into  the  duodenum 
at  points  widely  remote 
from  each  other. 

As  in  the  birds,  the 
heart  is  four-chambered ; 
the  divisions  occasionally 
are  visible  from  the  outside 
as  in  the  dugong.  Its  ma- 
jor axis  is  horizontal  except 
in  the  anthropoids  and  man. 
The  arch  of  the  aorta  bends 
to  the  left,  a  condition  ref- 
erable back  to  the  fact  that 
it  is  the  persistent  (fourth) 
primitive  arch  of  the  left 
side.  From  the  proximal 
portion  of  the  aorta  there  are  given  off,  first,  the  coronary 
arteries  (usually  paired),  which  go  to  the  walls  of  the  heart,  and 
then  the  subclavians  and  carotids,  the  arrangement  of  which 
shows  many  variations  ;  the  most  usual  condition  being  first  a 
right  arteria  anonyma,  dividing  later  into  right  subclavian  and 
the  two  carotids,  and  then  the  left  subclavian.  Other  arrange- 
ments can  be  seen  from  the  diagram.  In  all  cases  the  right 
subclavian  is  in  part  the  persistent  right  fourth  arch  of  the 
embryo.  The  internal  carotids  enter  the  cavity  of  the  brain 
either  through  the  periotic  (petrosal)  bone,  or  between  it  and 
the  base  of  the  skull. 


FIG.  352.  Heart  of  dugong,  after  Macal- 
lister,  showing  the  double  character  of  the 
ventricles,  l>,  e  ;  a,  d,  auricles  ;  c,  pulmonary 
aorta  ;  /,  systemic  aorta. 


3/0  CLASSIFICATION  OF   VERTEBRATES. 

In  the  venous  system  the  most  noticeable  points  are  the  pres- 
ence of  valves,  at  least  in  the  veins  of  the  extremities.  Both 
pre-  and  postcavae  empty  directly  into  the  right  auricle  without 
the  presence  of  a  sinus  venosus.  In  rodents,  monotremes,  and 
the  elephants,  two  precavae  occur.  Rete  mirabilia  are  frequent 
in  various  situations.  The  red  blood  corpuscles  are  anucleate 
and,  except  in  tylopoda  where  they  are  oval,  are  circular  disks. 

The  lymph  vessels,  which  contain  numerous  valves,  empty 
by  means  of  a  principal  or  thoracic  duct  into  the  precava  near 
the  subclavian  vein.  In  their  course  are  numerous  lymph 
glands.  Closely  related  to  the  lymph  system  are  a  couple  of 
masses  of  adenoid  tissue,  the  tonsils,  peculiar  to  the  mammalia, 
placed  at  the  entrance  of  the  pharynx. 

The  entrance  to  the  trachea  (glottis)  is  covered  by  a  fleshy 
fold,  the  epiglottis.  The  larynx  is  well  developed,  with  aryte- 


FiG.  353-  Modifications  of  the  aortic  arch  and  its  vessels  in  mammals,  from 
Wiedersheim.  Ao,  aorta ;  c,  carotid ;  s,  subclavian  ;  tb,  truncus  brachiocephalicus 
(anonyma);  tbc,  truncus  brachiocephalicus  communis. 

noid,  cricoid,  and  thyroid  cartilages  (p.  28),  and  these,  moved 
by  appropriate  muscles,  put  various  tensions,  etc.,  upon  the 
vocal  cords.  The  cartilaginous  tracheal  rings  are  usually  in- 
complete behind ;  the  trachea  itself  is  never  convoluted,  and  it 
divides  behind  into  two  bronchi,  with  occasionally  a  secondary 
bronchus  on  the  right  side.  Inside  the  lung  the  rule  is  a  single 
eparterial  bronchus  arising  above  the  entrance  of  the  pulmonary 
artery,  and  nine  hyparterial  bronchi  on  either  side.  Air  sacs 
never  occur. 

The  inspiration  and  expiration  of  air  is  effected  in  part  by 
the  intercostal  muscles,  which  by  their  action  alter  the  size  of 
the  thoracic  cavity,  and  in  part  by  a  transverse  muscular  par- 
tition, the  diaphragm,  which  divides  the  abdominal  cavity  of  the 
lower  vertebrates  into  an  anterior  or  pleural  cavity  containing 


MAMMALS.  371 

the  lungs,  and  a  peritonial  cavity  containing  the  other  viscera 
(see  p.  1 06).  This  diaphragm,  while  foreshadowed  in  some 
sauropsida,  is  only  developed  in  the  mammals. 

The  functional  kidneys  (metanephridia,  p.  122)  are  small 
compact  organs,  only  occasionally,  as  in  some  seals  and  whales, 
showing  lobulations.  The  ureters  leading  from  them  empty 
into  the  dorsal  part  of  the  urinary  bladder.  The  urethra  lead- 
ing from  the  bladder  either  empties  into  the  cloaca  (mono- 
tremes)  or  into  the  urogenital  sinus.  No  renal  portal  system 
occurs. 

In  the  monotremes  and  whales  the  testes  remain  in  their 
primitive  position  near  the  kidneys,  but  in  all  other  mammals 
they  sink  into  the  pelvis.  In  the  elephants  they  do  not  proceed 
farther ;  in  the  rodents,  bats,  and  some  insectivora,  they  emerge 
during  the  breeding-season  into  a  temporary  sac  or  scrotum, 
and  after  this  time  is  passed  are  retracted  again  by  a  cremaster 
muscle.  In  the  other  mammals  the  testes  remain  permanently 
in  the  scrotum  after  their  descent,  and  the  opening  through 
which  they  descended  closes.  The  cremaster  muscle  persists, 
but  with  more  limited  functions.  Closely  connected  with  the 
male  genitalia  are  the  prostate  and  Cowper's  glands,  the  ducts 
of  which  empty  into  the  genital  duct  (vas  deferens),  the  secre- 
tion being  added  to  the  spermatozoa,  rendering  the  whole  more 
fluid. 

The  ovaries  are  relatively  small,  and  are  always  abdominal 
in  position.  The  oviducts  have  their  inner  ends  wide,  the  inter- 
nal apertures  being  usually  fimbriate.  In  each  duct  three  re- 
gions occur,  (i)  a  somewhat  narrow  Fallopian  tube  leading  to 
(2)  a  uterus  with  muscular  walls,  and  (3)  an  external  canal  or 
vagina.  In  the  lower  mammals  the  ducts  of  the  two  sides  may 
remain  distinct,  but  in  the  higher  fusion  begins  at  the  lower 
end,  resulting  in  a  single  vagina  and  a  uterus,  which  shows 
more  or  less  clearly  traces  of  its  double  origin. 

In  the  monotremes  the  large  eggs,  covered  by  a  flexible 
calcareous  shell,  pass  to  the  exterior,  but  in  all  other  mammals 
the  embryo  passes  through  a  considerable  portion  of  its  devel- 
opment in  the  uterus,  and  is  brought  into  the  world  in  a  more 
or  less  perfect  condition. 


3/2  CLASSIFICATION  OF   VERTEBRATES. 

The  development  of  the  mammals  pursues  two  distinct  types. 
In  the  monotremes  there  are  eggs  which  are  laid,  and  which  un- 
dergo their  development  outside  the  body  of  the  mother,  as  do 
those  of  birds  and  most  reptiles.  These  eggs  are  large  (about 
two  centimetres  in  diameter).  They  have  a  large  yolk,  and  the 
segmentation  is  restricted  to  a  small  portion  of  it,  just  as  is  the 
case  in  the  sauropsida ;  i.e.,  they  are  meroblastic. 

In  all  other  mammals  the  egg  is  much  smaller,  even  micro- 
scopic in  size,  and  the  early  stages  of  development  are  passed 
inside  the  mother,  the  young  being  born  alive.  These  smaller 
eggs  undergo  a  total  segmentation,  all  parts  dividing  ;  i.e.,  they 
are  holoblastic.  During  this  process  the  egg  increases  greatly 
in  size  by  the  absorption  of  fluid  which  fills  the  central  cavity. 
As  a  result  the  egg  is  converted  into  a  large  sphere  (blastula) 
covered  by  a  single  layer  of  cells  except  at  one  pole,  where 
there  are  a  number  of  '  inner  cell-mass  cells  '  beneath  the  others. 
Concerning  these  layers  there  is  much  difference  of  opinion, 
due  to  the  great  difficulty  surrounding  the  subject.  According 
to  one  view  the  outer  cells  are  ectoderm,  the  inner  entoderm  ; 
according  to  another  the  inner  cell-mass  is  ectoderm,  the  outer 
entoderm,  while  a  third  view  sees  both  ectoderm  and  entoderm 
in  the  inner  cell-mass.  Certain  it  is  that  the  region  of  the  inner 
cell-mass  eventually  becomes  two-layered,  and  later  the  embryo 
is  outlined  here,  only  a  portion  of  the  blastula  being  utilized  in 
its  formation,  the  rest  forming  a  yolk  sac,  in  the  walls  of  which 
omphalomeseraic  vessels  are  developed  later,  although  no  yolk 
occurs. 

This  development  of  a  complete  yolk  apparatus,  as  well  as 
several  other  peculiarities,  is  to  be  explained  upon  the  hypothesis 
that  the  mammals  have  descended  from  forms  —  like  reptiles  or 
amphibia  —  which  were  oviparous,  and  the  embryo  had  to  de- 
pend upon  the  food  stored  up  in  the  yolk.  Subsequently,  as  a 
result  of  an  internal  development  and  a  supply  of  nourishment 
from  the  mother,  the  yolk  was  lost ;  but  heredity  has  caused 
certain  features  not  incompatible  with  uterine  development  to 
be  retained.  The  mechanism  by  which  this  nourishment  from 
maternal  sources  is  transferred  to  the  embryo  has  now  to  be 
outlined. 


MAMMALS.  373 

Like  the  other  amniotes  the  mammalian  embryo  forms  the 
foetal  structures  amnion,  serosa,  and  allantois  (p.  288).  Of 
these  the  serosa  is  the  outermost,  and  necessarily  comes  in  con- 
tact with  the  uterine  walls.  In  most  marsupials  the  develop- 
ment goes  little  farther.  From  the  uterus  is  secreted  nutrient 
fluid  which  passes  through  the  serosa  by  osmosis,  and  is  thence 
taken  up  by  the  embryo,  furnishing  it  with  the  material  for 
growth,  which  in  oviparous  forms  is  supplied  by  the  yolk. 

In  Perameles,  one  of  the  marsupials,  and  in  all  the  higher 
mammals,  a  more  intimate  union  occurs  between  the  embryo 
and  the  uterine  walls  in  the  following  manner.  From  the  sur- 
face of  the  serosa  (which  from  this  time  on  is  known  as  the 
chorion)  numerous  outgrowths  or  villi  are  formed.  These  villi 
are  variously  arranged  in  different  mammals.  They  may  be 
distributed  evenly  over  the  whole  chorionic  surface  (diffuse),  or 
they  may  be  collected  in  tufts  scattered  over  the  surface,  the 
intermediate  regions  of  the  chorion  being  smooth  (cotyledonary)- ; 
again,  they  may  form  a  girdle  around  the  chorion,  the  ends  being 
free  from  villi  (zonary)  ;  or,  lastly,  they  may  be  restricted  to  a 
more  or  less  circular  patch  on  one  side  of  the  chorion  (discoidal). 
These  villi  enter  into  more  or  less  intimate  connection  with  the 
uterine  walls  in  ways  to  be  described  below. 

The  allantois  (p.  289)  grows  out  from  the  body,  and  finally 
reaches  the  inner  surface  of  the  chorion,  carrying  with  it  the 
allantoic  blood-vessels.  The  union  of  chorion  and  allantois  is 
coextensive  with  the  development  of  the  villi  upon  the  outer 
surface,  and  the  resulting  structure  forms  the  embryonic  por- 
tion of  the  placenta.  The  blood-vessels  of  the  allantois  may  be 
confined  to  that  structure,  or  they  may  extend  out  into  the  cho- 
rion, but  in  either  case  they  carry  away  from  the  embryo  waste 
which  passes,  by  osmosis,  to  the  maternal  tissues,  and  at  the 
same  time  bring  back  to  the  growing  young  nourishment  and 
oxygen,  which  pass  into  the  foetal  blood  by  osmotic  action.  In 
no  case  is  there  a  direct  connection  between  maternal  and  foetal 
blood-vessels ;  but  the  exchange  is  always  of  the  character 
indicated  here. 

It  must,  however,  be  noted  that  the  relations  of  the  allantois 
to  the  chorion  follow  two  types.  In  the  unguiculate  mammals 


374 


CLASSIFICATION  OF   VERTEBRATES. 


the  allantois  early  grows  out  to  join  the  chorion,  and  brings 
with  it  its  blood-vessels,  which  then  ramify  through  the  chorion, 
which  therefore  has  its  own  circulation,  although  this  is  depen- 
dent upon  the  allantois.  In  the  ungulates  the  allantois,  although 

well  developed,  re- 
mains for  a  consid- 
erable time  distinct 
from  the  chorion,  and 
only  later,  when  its 
expansion  brings  it  in 
contact  with  the  lat- 
ter, does  the  chorion 
receive  its  vascular 
supply.  These  two 
types  are  known  re- 
spectively as  the  al- 
lantoic  and  chorionic 
placenta. 

In  many  mammals 
the  union  between  the 
villi  of  the  chorion 
and  the  uterine  walls 
is  slight,  and  at  the 
time  of  birth  the  two 
separate,  only  the  em- 
bryonic placenta  be- 
ing cast  off.  These 
forms — including  the 
ungulates,  cetacea,  si- 
renia — are  called  non- 
deciduata,  or  indecid- 
uata.  In  others  the 

union  is  far  more  intimate,  the  branched  villi  entering  into  such 
close  connection  with  the  uterus  1  that,  at  the  time  of  birth,  a 
portion  or  all  the  uterine  walls  (the  decidua)  is  cast  off  with 
the  embryonic,  or  foetal,  placenta.  In  some  mammals,  as  in  man, 
the  decidua  exhibits  certain  peculiarities.  At  the  time  of  at- 

i  This  forms  the  uterine  placenta. 


FIG.  354.  Diagram  of  human  uterus  and  placenta, 
based  on  Wiedersheim.  Foetal  parts  lined,  uterine 
dotted,  the  decidual  portions  darker.  A,  cavity  of 
the  amnion;  /',  foetal  placenta;  Z,  chorion  laeve;  7\, 
decidua reflexa;  6",  maternal  placenta  (decidua  sero- 
tina);  7',  entrance  of  Fallopian  tube;  F,  decidua 
vera. 


MAMMALS.  .375 

tachment  of  the  ovum  to  the  walls  of  the  uterus,  these  walls  rise 
up  over  and  enclose  the  egg,  thus  coming  in  contact  with  it  on 
all  sides.  From  but  one  side,  however,  is  the  (discoidal)  pla- 
centa developed,  and  in  this  region  the  decidua  is  spoken  of  as 
the  decidua  serotina,  while  that  which  covers  the  smooth  or  non- 
villous  portion  of  the  chorion  (chorion  laeve)  forms  the  decidua 
reflexa.  The  rest  of  the  uterine  walls,  which  do  not  connect 
with  the  ovum,  are  also  cast  off  at  birth,  and  these  form  the 
decidua  vera. 

In  older  books  the  eutheria  of  the  following  pages  are  fre- 
quently divided -into  the  Implacentalia,  including  the  marsupials, 
and  the  Placentalia,  including  the  remaining  orders  ;  but  the 
recent  discovery  that  at  least  one  genus  of  marsupials  (Pera- 
meles)  has  a  true  allantoic  placenta  tends  to  break  down  this 
line.  Still  the  distinction  is  one  of  convenience,  and  has  been 
used  in  these  pages,  the  term  placentalia  including  all  the  orders 
from  edentates  to  primates,  the  implacentalia,  the  marsupials, 
and  frequently  the  monotremes,  when  these  have  not  been 
specially  mentioned. 

There  are  two  views  as  to  the  origin  of  the  mammals  ;  the 
one  that  they  have  descended  from  the  theromorphous  reptiles, 
the  other  that  they  have  sprung  from  the  amphibia.  The  first 
of  these  receives  its  chief  support  from  paleontology.  The 
theromorphs  have  a  heterodont  dentition,  a  triple  occipital 
condyle  from  which  the  paired  condyles  of  the  mammals  can 
be  derived  by  a  suppression  of  the  basioccipital  portion,  as  well 
as  several  features  in  the  skeleton  of  the  limbs.  The  advocates 
of  this  view  suppose  that  the  quadrate  has  disappeared  in  the 
region  of  the  glenoid  fossa. 

The  amphibian  view  receives  its  support  in  the  double  occip- 
ital condyle,  the  impossibility  of  deriving  the  mammalian  ovum 
from  that  of  any  known  reptiles,  and  its  easy  homology  with 
those  of  amphibia,  and  in  the  relations  of  the  ear  bones.  This 
view  recognizes  the  quadrate  in  the  incus,  and  this  articulates 
with  the  stapes,  a  condition  repeated  in  the  urodeles,  but  not 
derivable  from  anything  known  in  the  reptiles  (see  p.  1 59). 
Another  difficulty  with  the  reptilian  hypothesis  is  the  impos- 
sibility of  deriving  the  mammalian  hair  from  any  exoskeletal 


376 


CLASSIFICATION  OF   VERTEBRATES. 


structures  occurring  in  reptiles  ;  while  there  are  several  features 
which  point  to  the  possibility  of  their  origin  from  the  dermal 
sense  organs  of  the  amphibia. 

SUB-CLASS   I.     PROTOTHERIA. 

Mammals  with  a  single  opening  for  urogenital  system  and 
alimentary  canal ;  sutures  of  skull  obliterated  in  the  adult ;  a 
well-developed  coracoid  and  episternum  ;  oviparous. 

ORDER   I.     MONOTREMATA    (ORNITHODELPHIA). 

Prototheria  with  small  corpus  callosum  and  large  anterior 
commissure ;  no  teeth  in  the  adult ;  epipubic  bones  present ; 
ribs  with  capitular  head  only ;  mammary  gland  without  distinct 
nipple. 

The  few  existing  species  of  monotremes  are  restricted  to 
the  Australasian  region,  and  the  only  fossils  certainly  belonging 
to  the  order  occur  in  the  pleistocene  of 
Australia.  These  mammals  are  remark- 
able for  the  large  number  of  sauropsidan 
features  which  they  present.  Besides 
the  characters  given  in  the  diagnosis  the  follow- 
ing may  be  added.  The  ossicula  auditus  are 
of  a  low  grade,  the  malleus  being  large  and  the 
stapes  columelliform.  In  the  embryo  of  the 
duckbill  multituberculate  teeth  occur,  but  these 
are  lost  before  maturity,  and  the  adults  of  all 
species  are  toothless.  Lips  are  lacking,  and  the 
jaws  form  horny  beaks.  The  brain  is  smooth 
in  Ornithorhynchus,  convoluted  in  Echidna,  The 
testes  are  abdominal  in  position  ;  the  left  ovary 
is  reduced  as  in  birds,  the  right  lobular.  There 
is  a  horny  perforated  spur  developed  on  the  hind  legs  in  con- 
nection with  a  gland.  This  spur  disappears  in  the  adult  female 
duckbill. 

Family  ORNITHORHYNCHIDJE.  With  duck-like  bill,  two  horny  teeth  in 
each  jaw;  feet  pentadactyl,  webbed;  tail  flat;  soft,  close  fur.  Ornitho- 
rhynchus  paradoxus,  the  duckbill  of  Australia  and  Tasmania,  is  the  only 


FIG.  355.  Em- 
bryonic teeth  of 
Ornithorhynchus, 
after  Stewart. 


MAMMALS.  377 

known  species.     It  leads  an  aquatic,  burrowing  life,  and  feeds  upon  worms 
and  small  aquatic  animalsr  using  its  bill  as  does  a  duck. 

Family  ECHIDNID^:.  Beak  elongate,  toothless;  tongue  elongate,  ver- 
miform ;  body  with  strong  spines  among  the  hair.  Echidna,  with  three 
species  from  Australia,  New  Guinea,  and  Tasmania,  has  all  the  toes  clawed. 
In  AcantJioglossus,  from  New  Guinea,  there  are  claws  on  but  three  toes, 
and  the  beak  is  longer.  All  of  these  spiny  ant-eaters  are  burrowing  animals, 
feeding  chiefly  upon  ants.  Echidna  occurs  as  a  fossil  in  the  Australian 
pleistocene. 


FIG.  356.     Duckbill,  Ornithorhynchus  paradoxus,  from  Liitken. 

The  earliest  fossil  mammals  yet  found  occur  in  the  triassic 
of  North  Carolina,  South  Africa,  and  Germany.  Little  is 
known  of  them  except  of  their  jaws  and  teeth.  Allied  forms 
are  more  abundant  in  later  rocks,  and  some  of  them  persist 
until  the  eocene.  From  peculiarities  of  the  teeth,  which  pre- 
sent certain  resemblances  to  the  embryonic  teeth  of  Ornitho- 
rhyncJnis,  these  fossils  are  sometimes  placed  as  members  of  the 
Prototheria,  an  example  followed  here ;  although  they  also 
present  resemblances  to  the  marsupials. 

ORDER    II.     PROTODONTA. 

Incisors  reduced,  molars  with  compressed  cutting  crowns 
and  undivided  roots.  Represented  only  by  lower  jaws  of  Droma- 
therium  and  Microconodon  from  the  American  Jurassic. 

ORDER    III.      MULTITUBERCULATA    (ALLOTHERIA). 

Incisors  enlarged,  molars  tuberculate  with  distinct  roots.  In 
these  forms,  which  are  represented  by  several  genera,  the  teeth 


CLASSIFICATION  OF   VERTEBRATES. 

are  very  numerous,  ranging  from  48  to  68.  Plagiaulax  from 
the  Purbeck  beds  (upper  Jurassic)  of  England ;  Ctenacodony 
American  Jurassic  ;  Chirox  and  Poly  mastodon  from  the  Puerco 
(lower  eocene)  of  America.  The  Australian  quaternary  Thy- 
lacoleo,  usually  regarded  as  a  marsupial,  may  belong  here. 

SUB-CLASS    II.     EUTHERIA. 

Mammals  with  anus  distinct  from  the  urogenital  opening ; 
sutures  of  the  skull  well  marked  ;  episternum  reduced  ;  coracoid 
not  articulating  with  the  sternum,  but  reduced  and  fused  with 
scapula ;  viviparous  ;  mammae  with  teats. 

Legion  I.     Didelphia. 

Eutherian  mammals,  with  small  corpus  callosum,  usually 
with  marsupial  bones  (except  in  Thylacinus).  Vaginae  partially 
or  completely  double.  As  a  rule  no  placenta  developed. 

ORDER    I.     MARSUPIALIA. 

Teeth  always  present,  only  one  (/3)  replaced  by  a  second 
dentition,  the  number  usually  different  in  upper  and  lower  jaws  ; 
two  precavae  present ;  mammae  abdominal  in  position  and  usually 
enclosed  in  a  pouch  in  which  the  very  immature  young  are  placed 
after  birth. 

The  order  marsupialia  and  the  legion  didelphia  are  coexten- 
sive. The  living  species  are  almost  exclusively  confined  to 
Australia  and  the  adjacent  islands  ;  the  only  exceptions  being 
the  family  didelphidae,  which  is  American.  Fossils,  however, 
are  found  in  Europe  as  well.  Forms  certainly  belonging  to  the 
order  first  occur  in  the  eocene  ;  but  others,  possibly  related,  date 
from  the  cretaceous.  The  order  receives  its  name  from  the 
pouch  (marsupium)  in  which,  in  most  species,  the  young  are 
carried  after  birth  ;  but  this  pouch  is  not  invariably  present,  the 
young  in  these  cases  being  held  in  the  fur  covering  the  abdom- 
inal region.  When  first  born  the  young  are  very  immature. 
They  are  transferred  by  the  mother  to  the  nipples,  to  which 
they  adhere  closely.  Milk  is  forced  into  their  mouths  by  mam- 
mary muscles,  and  strangulation  of  the  young  is  prevented  by  a 


.MAMMALS. 


379 


prolongation  of  the  larynx  into  the  choana,  much  as  in  the  whales. 
In  development  the  ova  pass  into  the  uterus,  from  which  they 
absorb  nourishment  without  the  intervention  of  a 
placenta  (except  in  Perameles),  no  villi 
being  developed  on  the  serosa,  and 
the  allantois  failing  to  reach  this 
envelope.     An  osteological  pe- 
culiarity, present  in  all  except 
in  Tarsipes,  is  the  inflection 
of  the  posterior  angle  of  the 
jaw. 

In  size  the  marsupials 
vary  from  animals  the  size 
of  a  rat,  up  to  the  giant 
kangaroo  ;  while  in  the  past 
Diprotodon  was  as  large  as 


FIG.  357.     Skeleton  of  kangaroo,  from  Macallister. 

a  rhinoceros.  In  form  and  habits  they  show  many  modifica- 
tions, usually  attributed  to  the  fact  that  in  Australia  they 
have  been  removed  from  competition  with  other  mammals,  and 
have  developed  in  every  direction,  —  terrestrial,  crawling,  leap- 
ing, climbing,  and  soaring  forms.  The  majority  are  nocturnal. 


SUB-ORDER  i.     POLYPROTODONTIA. 

Incisors  -? -,  small,  subequal ;  canines  larger ;  molars  acutely  tuber- 

4  or  3 
culate. 

The  DIDELPHID.E,  opossums,  American  ;  teeth  i  \,c\tp\ym%\  feet  all 
five-toed  ;  tail  partially  naked  and  usually  prehensile.  Didelphys  virginiana* 
north  to  New  England  ;  other  species  in  the  tropics.  Chironectes  has  webbed 
feet.  Didelphys  occurs  in  the  eocene  of  France  and  America.  DASYURHXE 


-,c-,p~  -  -,m-—;\    hind    feet    four-toed. 
3       i  ^  2  or  3         4-6  ' 


Thy  I  acinus    is    dog-like, 


carnivorous,  and  occurs  in  Tasmania.  Myrmecobius  with  m  f  ,  feeds  on 
ants.  Dasyurus,  Phascogale.  Allied  forms  fossil  in  lower  tertiary  of  South 
America  and  later  tertiary  of  Australia.  The  PERAMELID^E,  z  f  ,  c  \,  p  f  * 


380 


CLASSIFICATION  OF   VERTEBRATES. 


.m  £ ,  include  the  genera  Perameles,  in  which  the  feet  are  much  alike,  and 
^Chczropus,  in  which  the  hind  legs  are  very  long  and  the  fourth  toe  alone 
functional.  The  bandicoots  (Peraweles)  are  no- 
ticeable from  the  existence  of  a  placenta.  Fossils, 
which  in  some  respects  closely  resemble  the  polypro- 
todonts  and  in  some  the  insectivores,  are  the  TRI- 
CONODONTA  and  TRITUBERCULATA,  with  the  genera 
Amphilestes,  (Jurassic,  England  and  the  U.  S.), 
Dicrocynodon  (Jurassic,  Wyoming),  Amphitherim 
(English  oolite),  Dryolestes  (Jurassic,  Wyoming), 
etc. 


FIG.  358.  Opossum, 
JDidelphys  virginiana, 
after  Audubon  and 
JBachman. 


SUB-ORDER  2.     DIPROTODONTA. 


Incisors  _,  the  central  ones  large,  the  others 


reduced ;    canines    small    or   absent ;    molars    with 
blunt  tubercles  or  transverse  ridges. 
In  the  kangaroos  and  wallabies  (MACROPODID/E)  the  hind  legs  are  very 

large;  the  feet  as  in  Perameles  :  the  teeth   /  _,  c     or_,  ft  ,  m  _;    tail 

i       o      o  y  2  or  i         4' 

very  large.  The  larger  kangaroos  belong  to  Macropus ;  the  arboreal  tree- 
kangaroos  to  Dendrolagus.  Macropus,  Palorchestes,  etc.,  occur  in  Austra- 
lian pleistocene.  The  PHALANGISTID^E  includes  climbing  and  flying 

'(soaring)  forms,  with  legs  of  equal  size,  teeth  i  .  ,  c  -,p — -,  in  -,    tail    long. 

I         O         2—1  4 

Tarsipes  is  an  aberrant  form  about  as  large  as  a  mouse.  Pet  aunts,  Be- 
lidius,  etc.,  resemble  the  flying-squirrels  in  the  lateral  fold  of  skin  and 
ilying  habits.  Cuscus  and  Phalangista  resemble  the  opossums  in  their 
prehensile  tail.  Phascolarctos,  the  koala,  contains  but  a  single  climbing 
.species  two  feet  long.  The  THYLACOLEONID^E  includes  large  fossil  forms 
from  the  Australian  pleistocene,  with  teeth  *  f ,  c  ^,  p  f ,  ;//  \.  The  kan- 
garoo-rats, or  HYPSIPRYMNID/E,  with  teeth  i  |,  c  \,  p  \  m  \,  resemble  the 
kangaroos  in  the  disproportionate  hind  legs.  Hypsiprymnus,  Bettongia, 
the  last  also  in  the  Australian  pleistocene.  The  DIPROTODONTID^:  in- 
cludes only  fossil  forms  of  large  size  from  the  Australian  pleistocene,  with 
the  teeth  /  f ,  c  §,  p  \,  m  |.  Diprotodon  australis  was  larger  than  a  rhi- 
noceros;  the  species  of  Notothenum  somewhat  smaller.  The  PHASCOLO- 
MYID^E,  with  a  dental  formula  i  \,  c  ^,  p  \,  m  |,  differ  from  all  other  mar- 
supials in  the  presence  of  persistent  dental  pulps.  The  living  wombats  all 
belong  to  Phascolomys,  which  also  occurs  in  the  pleistocene.  The  extinct 
Phascnlonus  was  as  large  as  a  tapir.  South  America  has  yielded  several 
fossil  diprotodonts  of  eocene  or  miocene  age,  and  one  recent  species, 
^Ccenolestes  obscurus,  has  been  described  from  Colombia. 


MAMMALS.  381 

Legion  II.     Monodelphia    (Placentalia). 

Eutherian  mammals  with  well-developed  corpus  callosumr 
and  small  anterior  commissure  ;  no  marsupial  bones  ;  vagina 
single ;  foetus  nourished  by  an  allantoic  placenta. 

ORDER   I.     EDENTATA  (BRUTA). 

Placental  mammals  with  the  incisors,  and  occasionally  all 
the  teeth,  lacking.  Teeth  when  present,  usually  prismatic ; 
molars  without  enamel.  Carpals  and  tarsals  usually  in  linear 
series  (taxeopodous,  p.  392)  ;  digits  armed  with  long,  com- 
pressed, and  pointed  claws. 

The  Edentata  includes  a  rather  heterogeneous  assortment 
of  forms,  the  range  of  variation  being  even  greater  when  the 
fossils  are  considered.  Most  of  the  species  are  not  strictly 
edentulous,  since  molars  are  usually  present.  These  are  homo- 
dont,  and  except  in  Tatusia  they  are  monophyodont  and  have 
persistent  pulps.  The  skin  is  covered  with  hair,  horny  scales, 
or  bony  shields,  these  sometimes  uniting  into  a  more  or  less 
complete  armor  for  the  body.  The  mammae  are  thoracic  or 
abdominal  in  position.  The  cerebral  hemispheres  are  small. 
The  placenta  shows  great  variations ;  it  may  be  deciduate  or 
not  ;  in  shape  it  may  be  diffuse,  discoidal,  or  of  discoidal  lobes,, 
or  zonary. 

The  edentates  are  given  a  position  here  at  the  base  of  the- 
placental  mammals  because  of  their  low  grade  of  structure- 
In  some  respects,  as  in  the  simple  condition  of  the  brain,  this 
low  grade  is  primitive  ;  but  in  other  respects,  as  in  skeleton 
and  teeth,  the  group  is  clearly  degenerate,  although  as  yet  it 
is  uncertain  from  what  group  they  have  sprung.  According  to* 
Cope  they  have  probably  descended  from  the  group  of  tillo- 
dontia  of  the  later  cretaceous  and  eocene.  The  earliest  fossil 
edentates  known  occur  in  the  Santa  Cruz  beds  of  Patagonia, 
regarded  by  Ameghino  as  eocene,  but  by  some  as  oligocene  ; 
and  it  is  interesting  to  note  that  these  early  forms  retained 
traces  of  enamel  upon  the  teeth. 

The  group,  as  a  whole,  belongs  to  the  tropics  and  the  south- 


382  CLASSIFICATION-  OF   VERTEBRATES. 

ern  hemisphere,  but  few  species  straying  north  of  the  tropic  of 
cancer.  In  times  past  they  had  a  greater  range  ;  for  while  the 
centre  even  then  was  in  the  south,  a  few  species  occurred  as 
far  north  as  southern  Europe,  and  to  latitude  46°  in  the  new 
world.  The  American  forms  differ  from  those  of  the  old  world 
In  the  existence  of  articular  processes,  besides  the  normal  zyga- 
pophyses  on  the  presacral  vertebrae.  These  have  therefore 
been  called  Xenarthra  in  contradistinction  to  the  Nomarthra 
of  the  eastern  hemisphere.  To  the  Nomarthra  belong  the 
sub-orders  Fodientia  and  Squamata ;  the  other  sub-orders  are 

xenarthrous. 

SUB-ORDER  i.     FODIENTIA. 

Body  covered  with  sparse,  bristle-like  hairs,  five  prismatic  molars  in 
each  jaw ;  femur  with  a  third  trochanter,  toes  four  in  front,  five  behind ; 
placenta  zonular.  Only  the  single  family,  ORYCTEROPODID^E,  containing  the 
aardvark,  Orycteropus  capensis,  of  South  Africa  and  a  fossil  species  from 
the  miocene  of  the  island  of  Samos.  The  aardvark  lives  a  burrowing  life, 
feeding  upon  ants  and  other  insects.  It  is  about  as  large  as  a  pig. 


iro. 


FlG.  359.      Pangolin,  Manis  longicaudata,  from  Montei 

SUB-ODER  2.     SQUAMATA. 

Body  covered  with  overlapping  horny  scales  and  scattered  hairs  ;  jaws 
toothless  ;  tongue  long,  vermiform  ;  feet  five-toed  ;  placenta  non-deciduate, 
diffuse.  Contains  the  single  family  MANID^E,  the  scaly  ant-eaters  or  pango- 
lins of  Asia  and  Africa.  Only  genus  Mam's,  which  also  occur  fossil  in  the 
pleistocene  of  Asia.  All  the  species  are  arboreal  and  insectivorous ;  and 
have  a  somewhat  reptilian  appearance  on  account  of  the  scaly  body  and 
long  tail. 


MAMMALS. 


383 


SUB-ORDER  3.     VERMILINGUIA. 


Body  hairy  ;  skull  very  long  ;  no  teeth  ;  tongue  very  long  and  mobile  ; 
tail  elongate  ;  hind  feet  five-toed  ;  placenta  deciduate,  dome-like  or  dis- 
coidal.  The  ant-eaters  form  the  family  MYRMECOPHAGID^E,  all  of  which 
live  in  tropical  America,  where  they  feed  upon  ants  and  other  insects  ;  a  few 
are  arboreal.  Myrmecophaga  jubata,  the  great  ant-eater,  five  feet  long,  lacks 
a  claw  on  the  fifth  fore  toe.  In  Cyclotura  only  digits  2  and  3  are  clawed. 
Fossils  (Scotaops),  supposed  to  belong  to  this  sub-order  but  with  two 
molar  teeth,  occur  in  the  Patagonian  eocene. 

SUB-ORDER  4.     TARDIGRADA. 


Body  haired  ;  head  short  and  rounded  ;   molars    _.     The  BRADYPO- 

4  or  3 

,  or  sloths,  have  cylindrical  teeth  ;  short,  weak  tail  ;  long,  slender 
limbs  ;  digits  armed  with  long,  strong  claws  ;  and  deciduate,  dome-like  pla- 
centa with  numerous  discoidal  lobes.  Bradypus,  the  threfe-toed  sloths  ; 
Chol&pus,  the  two-toed  sloths.  Both  genera  have  the  hind  feet  three-toed, 
and  are  noticeable  for  the  number  of  cervical  vertebrae  (p.  355).  They  are 
arboreal,  and  live  almost  entirely  in  the  trees.  Entelops  from  the  eocene  of 
Patagonia.  The  extinct  MEGATHERIID/E  includes  giant  edentates  from  the 
pleistocene  of  both  Americas.  They  had  prismatic  teeth  of  peculiar  struc- 
ture ;  large,  long  tails  and  stout  limbs  ;  feet  3-5  toetl.  Megatherium  from 
South  America,  and  one  doubtful  species  from  the  U.  S.  The  largest  species 
equalled  an  elephant  in  size.  Megalonyx,  first  described  by  Thomas  Jeffer- 
son, and  My  lotion  ranged  north  to  Pennsylvania.  Zamicrus,  Patagonian 
eocene.  There  is  some  evidence  that  a  species  of  Mylodon  (Neomylodon) 
still  persists  in  Patagonia. 

SUB-ORDER  5.     LORICATA. 
Body  with    armor  of  bony  plates  ;    teeth    prismatic,    usually  —  -  -- 

GLYPTODONTID^E  ;  trunk  plates  united  into  a  solid  carapace,  with  other  plates 
on  the  tail  ;  dorsal  vertebrae  fused  to  a  continuous  tube.  Tertiary  and 
pleistocene  of  South  America  and  north  to  U.  S.  Glyptodon,  Hoplophoms, 
Parocthus.  These  resembled  turtles  in  appearance.  One  species  12  feet 
long.  DASYPODID^E,  dermal  armor  in  three  or  more  movable  transverse 
rows,  vertebrae  free.  These  armadillos  first  appear  in  the  Patagonian  eocene, 
and  continue  until  the  present.  The  living  species  are  small,  nocturnal,  car- 

o 

nivorous  forms.     Chlamydophorus  ;  teeth  _  —  ;  armor  of  about  20  trans- 

8  to  9 

verse  bands  :    body  truncate  behind.     Dasyfius  ;  teeth   -5  —  -  —  .  ;    armor  of 

10  to  9 

two  shields  upon  scapular  and  pelvic  regions,  with  six  or  seven  bands  be- 
tween. Xolypeutes  with  three  bands  ;  Xennrus  with  twelve  or  thirteen. 

Tatusia  has  0  to  7  teeth,  all  except  the  last  preceded  by  milk  dentition  ; 

o  to  7 


384  CLASSIFICATION  OF   VERTEBRATES. 

seven  to  nine  movable  armor  bands.  Tatusia  novemcincta  is  the  only  arma- 
dillo entering  the  U.  S.  Chlamydotherium  from  the  pleistocene  of  Florida 
and  Patagonia  stands  nearest  the  glyptodonts.  Peltephilus,  eocene  of 
Patagonia. 


FIG.  360.     Nine-banded  armadillo,  Tatusia  novemcincta,  from  Liitken. 

ORDER   II.     INSECTIVORA. 

Small  plantigrade  mammals,  usually  with  five  toes  armed 
with  claws  ;  carpals  and  tarsals  usually  in  linear  series ;  denti- 
tion complete,  the  incisors  never  less  than  two  ;  canines  little 
differentiated  and  weak ;  molars  bunodont  or  lophodont,  the 
cusps  acute ;  clavicles  almost  invariably  present ;  brain  small, 
cerebrum  without  convolutions  ;  placenta  deciduate,  discoidal. 

The  insectivores  owe  their  name  to  the  fact  that  the  major- 
ity feed  upon  insects  or  other  small  invertebrates.  They  are  all 
small,  and  the  structure  points  to  a  low  stage  of  organization. 
The  body  is  covered  with  fur,  and  spines  are  not  infrequently 
developed.  The  milk  dentition  is  lost  at  an  early  date,  and 
rarely  is  functional.  The  canine  teeth  are  not  sharply  differen- 
tiated from  the  incisors  or  premolars,  and  the  latter  are  sharp 
sectorial.  The  upper  molars  have  three  or  four  cusps.  The 
testes  are  internal,  and  are  never  enclosed  in  a  scrotum  ;  the 
uterus  is  bicornuate.  In  a  few  genera  vertebral  intercentra 
occur  in  the  dorsal  region,  a  condition  not  paralleled  in  other 
mammalia.  Among  the  more  superficial  but  still  very  charac- 
teristic features  is  the  prolongation  of  the  muzzle  far  beyond 
the  lower  jaw. 

Most  of  the  order  are  nocturnal  burrowing  animals,  only  a 
few  being  aquatic  or  arboreal  in  habits.  In  external  appearance 
they  resemble  the  smaller  rodents  ;  but  in  structure  they  are 
more  like  the  bats,  with  other  resemblances  to  the  polyproto- 


MAMMALS.  385 

dont  marsupials,  the  creodont  carnivores,  and  the  lemurs.  They 
inhabit  to-day  only  the  old  world  and  North  America  ;  while  the 
fossils  occur  only  in  the  northern  hemisphere,  where  they  date 
back  to  the  eocene.  The  order  is  one  of  the  most  primitive  of 
the  placental  mammals  ;  but  as  yet  the  fossils  are  too  few  and 
too  imperfectly  preserved  to  allow  the  complete  working  out  of 
the  lines  of  descent.  As  here  limited  the  order  includes  only 
the  Insectivora  Vera.  By  some  writers  the  galago,  Gateopithecus, 
of  the  East  Indies  (seep.  415)  is  included  in  a  second  sub-order, 
Dermaptera. 

The  ICTOPSID^E,  from  the  American  eocene  and  miocene,  have  skulls 
much  like  the  hedgehogs,  but  a  simpler  dental  pattern.  The  ADAPISORI- 
CID,E  take  their  place  in  the  eocene  of  France.  The  TALPID^E,  or  moles, 
with  /  |  to  f,  c  \,  PHI  %  to  f ,  ;//  f ,  snout  elongate,  tympanic  bulla  present ; 
fore  limbs  modified  for  digging,  with  a  sesamoid  bone  (os  falciforme)  on  the 
radial  side  ;  tibia  and  fibula  united ;  are  small  burrowing  animals,  of  which 
Talpa  is  the  typical  genus,  with  /  f ,  c  \,  p  |,  m  f .  The  species  of  Talpa 
belong  to  the  temperate  part  of  the  old  world.  In  America  occur  the 
genera  Scalops,  with  36  teeth,  and  Condylura,  the  star-nosed  mole,  with  44 
teeth.  Talpa  dates  from  the  miocene,  Talpavus  from  the  miocene.  Allied 
are  the  MYOGALID.E,  in  which  the  falciform  bone  is  absent.  Urotrichus, 
the  mole  shrew,  is  the  only  North  American  genus.  In  the  TUPAIID^E,  in 
which  the  lower  incisors  are  never  less  than  two,  the  tibia  and  fibula  are 
distinct,  and  the  orbit  is  encircled  by  bone.  The  species  are  oriental  in 
their  distribution,  and  have  arboreal  habits.  Titpaia.  Galerix,  from  the 
European  eocene.  The  shrews  (SORICID^E),  which  appear  in  the  eocene, 
are  distributed  through  the  northern  hemisphere.  They  lack  the  postorbital 
process,  have  tibia  and  fibula  fused,  and  no  zygomatic  arch  ;  teeth  i  \  to  f , 
c  ^,  p  — ^-,  m  \.  Sorex\s  represented  by  many  species  in  both  hemispheres. 
Blarina  is  American.  Crocidura,  Nectogale.  The  DIMYLID.E  includes 
miocene  species.  In  the  ERINACEID^E,  or  hedgehogs,  the  dorsal  surface 
is  covered  with  spines  or  bristles.  All  of  the  species  belong  to  the  old 
world,  and  are  terrestrial  and  nocturnal.  The  hedgehogs  belong  to  Erina- 
cens,  a  genus  which  appears  in  the  miocene.  The  species  of  MACROSCEL- 
ID/E  from  Africa  are  known  as  the  jumping  shrews,  from  their  kangaroo-like 
gait.  The  SOLENODONTID^E  from  the  West  Indies  are  remarkable  for  having 
the  mammae  on  the  buttocks.  The  tenrecs  (CENTETID.E)  are  from  Mada- 
gascar. The  golden  moles  (CHRYSOCHLORID/E)  of  Africa  have  the  hair  of  a 
brilliant  metallic  lustre,  bronze,  green,  or  violet  in  color ;  the  eyes  are  cov- 
ered by  the  integument,  and  the  external  ears  are  concealed  by  the  fur.  The 
last  four  families  have  no  fossil  representatives,  but  are  nearest  in  structure 
to  the  Ictopsidas. 


386 


CLASSIFICATION  OF   VERTEBRATES. 


ORDER   III.     CHIROPTERA. 

Flying  mammals,  in  which  the  anterior  limbs  are  modified 
into  supports  for  the  membranous  wings  ;  dentition  complete, 
the  canines-  strong,  the  molars  buno-lophodont ;  the  total  teeth 
never  exceeding  i  |,  c  1,  /  |,  m  | ;  mammae  pectoral ;  testes 
abdominal  or  inguinal  ;  placenta  discoidal,  deciduate. 

The  bats  must  be  regarded  as  highly  specialized  offshoots 
from  the  insectivores,  with  which  they  closely  agree  in  all 
essential  points  except  the  development  of  wings.  These  last 


FIG.  361.     Skeleton  of  bat,  after  Brehm. 

are  membranous  folds,  supported  upon  a  bony  framework  com- 
posed of  the  modified  fore  limbs  and  extending  back  to  the 
hind  legs,  while  an  interfemoral  membrane  may  or  may  not 
include  the  tail  when  this  is  developed.  Muscles  to  move  the 
wing  are  attached  to  the  sternum,  which  develops  a  keel  similar 
to  that  of  the  birds.  The  modifications  in  the  fore  limbs  con- 
sist in  an  enormous  lengthening  of  the  digits  with  the  exception 
of  the  pollex,  which  remains  more  normal,  and  may  terminate 
with  a  claw. 

The  bones  are  very  light,  being  slender,  with  large  marrow 
cavities  ;  the  skull  varies  considerably,  and  usually  possesses  a 


MAMMALS.  387 

complete  zygomatic  arch,  while  as  generally  there  are  no  frontal- 
postorbital  processes.  The  clavicle  is  present,  and  the  ulna  is 
rudimentary.  The  brain  is  small  and  smooth.  The  sense  of 
touch  is  highly  developed,  the  wings  being  important  in  this 
respect,  while  in  many  species  a  peculiar  dermal  sensory  appa- 
ratus, the  '  nose-leaf,'  is  developed  upon  the  snout.  The  shape 
of  this,  as  well  as  that  of  the  ears,  is  very  variable,  and  is  util- 
ized in  classification.  The  teeth  are  closely  similar  to  those  of 
the  insectivores  ;  the  milk  dentition  is  poorly  developed,  and 
in  some  instances  is  lost  before  birth.  The  intestine  is  short  — 
shortest  in  the  insectivorous  species  ;  a  caecum  rarely  occurs. 
The  left  lateral  lobe  of  the  liver  is  very  large,  and  a  gall  bladder 
is  present. 

About  four  hundred  species  of  bats  are  known,  all  nocturnal, 
and  usually  gregarious  in  their  habits.  Frequently  the  colonies 
are  found  to  be  composed  of  individuals  of  one  sex,  the  sexes 
only  coming  together  at  the  breeding-season.  There  is  some 
evidence  to  .show  that  the  males,  at  least  in  certain  species,  take 
part  in  nursing  the  young.  Fossils  are  rare  ;  they  first  appear 
in  the  eocene.  No  fossil  frugivora  are  known. 


SUB-ORDER  i.     ANIMALIVORA  (MICROCHIROPTERA). 

Small  bats  with  acutely  cuspidate  molars,  index  finger  reduced,  usually 
with  a  single  phalanx,  no  claw ;  stomach  simple,  intestine  short ;  outer  and 
inner  edges  of  ear  not  meeting  below ;  tail,  when  present,  connected  with 
the  interfemoral  membrane. 

The  old-world  RHINOLOPHID.E,  with  a  nose-leaf,  i  \,  p  f  or  f ,  and  a 
long  tail,  includes  about  fifty  species.  Rhinolophus  occurs  in  Europe, 
Asia,  and  Africa ;  Hipposiderus,  Asiatic.  Rhinolophus  occurs  in  the 
eocene  of  France.  Closely  allied  are  Nycteris  and  Megadenna  of  Asia,  in 
which  a  tragus  is  developed  in  the  ear.  The  VESPERTILIONID.E  have  a 
long  tail,  lack  the  nose-leaf,  and  have  a  tragus  to  the  ear  and  a  variable 
number  of  teeth.  Plecotus,  with  an  American  representative,  has  i  f,  c  |, 
p,  |  m  f ;  Antrozous,  from  California,  i  \,  c  \,  p  \,  m  f.  Vesperugo,  the 
largest  genus  of  bats,  is  cosmopolitan,  one  species  (  V.  serotinns}  inhabiting 
both  continents.  The  teeth  are  /  f  or  i,  c  \,  p  f  to  i,  ;//  f .  Atalapha  (i  |, 
c  1'  P  I  or  \->  m  f)'  exclusively  American.  Vespertilio  (i  f ,  c  -J-,  p  -|,  m  f), 
cosmopolitan.  Vesperugo,  eocene,  Wyoming.  Thyroptera,  Brazil.  KM- 
BALLONURID^:,  tropical  or  subtropical,  the  middle  upper  incisors  large  and 
close  together,  no  nose-leaf;  a  distinct  tragus,  and  obliquely  truncate  muz- 


388 


CLASSIFICATION  OF   VERTEBRATES. 


zle.  Emballonura,  old-world  tropics ;  Noctilio  and  Molossns,  tropical 
America.  PHYLLOSTOMID^E,  tropical  America;  have  three  phalanges  to 
the  middle  finger,  nose-leaf  present,  tragus  well  developed.  Chilonycteris, 
Vampyrus,  Glossophaga.  Desmodus  includes  the  blood-sucking  or  true 
vampire  bats. 


SUB-ORDER  2.     FRUGIVORA  (MEGACHIROPTERA). 

Large  bats  with  smooth-crowned  quadrituberculate  molars,  index  finger 
with  three  phalanges,  clawed ;  sides  of  the  ear  connected  below ;  tail,  when 
present,  below  the  interfemoral  membrane  ;  fruit-eating. 

The  only  family  is  the  PTEROPODID^E  of  the  East  Indies;  the  species 
of  which  are  generally  known  as  flying-foxes.  About  70  species,  40  being 
included  in  Pteropus. 


ORDER  IV.     RODENTIA    (GLIRES). 

Placental  mammals,  with  the  extremities  bearing  claws,  or 
more  rarely  hoof-like  nails  ;  feet  plantigrade  or  subplantigrade, 
usually  pentadactyl ;  condyle  of  lower  jaw  moving  in  an  elon- 
gate glenoid  fossa;  teeth  diphyodont  ;  canines  absent  ;  incisors 
long,  \  or  f ,  with  persistent  pulps  ;  molars  (including  premolars) 
varying  from  f  to  f  ;  placenta  discoidal,  deciduate. 


FIG.  362.     Skull  of  muskrat,  Fiber  zibithecus. 

The  rodents  are  as  sharply  marked  off  from  the  other 
mammals  as  are  the  sirenians  or  whales  ;  no  forms,  living  or 
fossil,  serving  to  connect  them  with  the  other  orders,  unless, 
possibly,  with  the  tillodontia.  Especially  characteristic  of  the 
group  are  the  gnawing  incisors,  in  which  the  enamel  is  on  the 
anterior  face,  the  resulting  wear  keeping  these  constantly  with 


MAMMALS.  389 

a  chisel-like  edge  ; 1  the  persisting  pulp  renews  all  loss  by  wear. 
Between  the  incisors  and  the  molars  is  a  wide  gap  or  diastema. 
The  molars  may  be  lophodont,  bunodont,  or  prismatic.  As  a 
whole,  the  dentition  varies  between  i  \ ,  c  §,  /  f,  m  f  (hares) 
and  i  \y  c  $,  p  #,  m  f ;  the  most  usual  being  i  \,  c  %,  p  §,  m  §. 
The  milk  dentition  does  not  include  the  incisors  as  a  rule,  and 
in  some  cases,  as  the  guinea-pigs,  is  lost  before  birth. 

The  skin  may  be  covered  with  the  softest  fur  (chinchillas), 
or  certain  hairs  may  be  developed  into  enormous  spines,  as  in 
the  porcupines ;  or  again,  the  spines  may  be  flattened ;  not 
infrequently  are  there  scales  on  the  tail.  Sternebrae  occur  in 
the  sternum.  The  skull  usually  presents  an  interparietal  bone; 
the  nasal  bones  are  large  and  long  ;  the  orbits  and  the  temporal 
fossae  are  confluent,  and  especially  characteristic  is  an  infra- 
orbital  canal  through  the  zygomatic  process  of  the  maxilla. 
The  clavicle  may  be  present  or  absent ;  the  manus  is  usually 
pentadactyl,  but  the  thumb  may  be  reduced,  while  in  the  hind 
foot  both  hallux  and  minimus  may  be  lost. 

Usually  (except  myoxidae)  there  is  a  large  intestinal  caecum; 
the  brain  is  small,  and  the  cerebral  hemispheres,  which  never 
cover  the  cerebellum,  are  smooth.  The  testes  are  inguinal  or 
abdominal  in  position,  while  there  is  either  a  uterus  bicornis  or 
two  distinct  uteri.  The  mammae  vary  extremely  between  the 
two  found  in  guinea-pigs  and  the  ten  in  some  rats. 

About  nine  hundred  living  species  of  rodents  are  known, 
and  they  occur  in  all  regions  of  the  world  except  the  Australian. 
They  are  mostly  small  and  are  mostly  arboreal ;  although  terres- 
trial, burrowing,  and  aquatic  species  occur.  All  are  herbivorous. 
The  order  appears  in  the  eocene,  but  has  its  greatest  develop- 
ment in  the  present  time.  The  genera  are  not  equivalent  to 
those  in  the  preceding  orders. 

SUB-ORDER  i.     SCIUROMORPHA. 

One  incisor  in  the  upper  jaw,  molars  f ;  clavicle  present ;  tibia  and  fibula 
distinct.  Mostly  belong  to  the  northern  hemisphere,  where  they  appeared 
in  the  eocene. 

1  Similar  conditions  exist  among  the  diprotodont  marsupials,  in  Typotherium  and  in 
some  multituberculates. 


390  CLASSIFICATION  OF   VERTEBRATES. 

SCIURID^E  ;  molars  f  or  |,  fore  feet  four-toed,  hind  pentadactyl ;  tail 
covered  with  fur.  To  the  family  belong  the  woodchucks  (Arctomys}, 
prairie  dogs  (Cynomys},  gophers  (Spermophilus} ,  chipmunks  (Tamms}, 
squirrels  (Sa'urus),  and  flying-squirrels  (Pteromys  and  Sciuropterus) ,  the 
latter  sailing,  rather  than  flying,  through  the  air  by  means  of  an  inter- 
membral  membrane  on  either  side  of  the  body.  Sciurus  appears  in  the 
eocene.  The  CASTORID^E,  or  beavers,  the  habits  of  which  are  so  well 
known,  have  the  molars  £,  the  feet  webbed,  and  the  tail  flattened  and 
scaly.  Castor  fiber,  the  beaver,  formerly  ranged  over  the  northern  parts  of 
both  continents,  but  has  been  greatly  restricted.  The  genus  dates  from 
the  pliocene  ;  the  allied  Stenofiber  is  miocene.  The  small  families  HAPLO 
DONTIDJE  and  ANOMALURID.E  are  represented  by  Haplodon,  the  sewellel 
of  Oregon,  and  Anomalurus,  a  flying  squirrel-like  form  from  Africa.  The 
fossil  family,  ISCHIROMYID^E,  occurs  in  the  eocene  and  miocene  of  North 
America. 

SUB-ORDER  2.     MYOMORPHA. 

Incisors  ^,  molars  f  or  f ;  clavicle  usually  present;  tibia  and  fibula 
fused.  The  DIPODID^E,  or  jumping  mice,  including  Zapus  of  the  United 
States,  Dipus,  the  jerboas  of  Europe,  and  Pedetes  of  South  Africa,  have 
the  hind  legs  long,  the  toes  being  5,  3,  and  4  respectively,  in  the  three 
genera.  A  much  larger  family  is  the  MURID^E,  in  which  there  are  no  pre- 
molars,  the  molars  are  f  to  f ,  and  the  tail  generally  naked  and  scaly.  Over 
three  hundred  living  species  are  known.  Cricetus,  including  the  hamsters 
of  the  old  world,  and  Hesperomys,  the  white-footed  mice  of  the  new,  have 
the  molars  f .  In  Arvicola  and  its  allies  the  tail  is  round,  and  the  molars 
rootless.  These  are  commonly  known  as  field-mice  or  voles.  The  migra- 
tory lemmings  of  northern  Europe  belong  to  Myodes.  Fiber  includes  our 
muskrat.  In  Mns,  which  contains  our  mouse  (M.  Hiusculus*),  and  our  rats 
(M.  decumanus,  the  brown  rat,  and  M.  rattns,  the  black  rat,  the  latter 
driven  out  by  the  former),  the  incisors  are  narrower  and  the  molars  rooted. 
The  family  dates  back  to  the  later  pliocene.  The  MYOXID^E  of  Europe, 
represented  to-day  by  the  seven-sleeper,  Myoxus  gtis,  dates  from  the  eocene. 
The  GEOMYID^:,  or  pocket  gophers,  receive  their  name  from  the  enormous 
cheek  pouches.  The  legs  are  fitted  for  burrowing,  and  the  molars  £•  Geo- 
mys  and  Thomomys  occur  in  our  central  region.  Farther  west  is  Sacco- 
mys  with  much  more  delicate  skull.  BATHYERGID/E  :  Spalax,  the  blind 
mole-rat  of  southeastern  Europe,  and  Bathyergus,  the  strand-rat  of  South 
Africa.  Lophiomys,  a  peculiar  arboreal  rat  with  hairy  tail,  from  north- 
eastern Africa,  is  nearest  to  Cricetus. 


SUB-ORDER  3.     HYSTRTCOMORPHA. 

Skull  with  very  large  infraorbital  canal ;  teeth  /  \,  c  §,  /  \,  m  f ;  zygo- 

matic  process  large,  clavicles  perfect  or  imperfect ;  tibia  and  fibula  separate. 

The  hystricomorphs  appear  in  the  eocene  of  Europe  and  South  Amer- 


MAMMALS. 


391 


ica,  the  latter  region  containing  the  majority  of  the  sub-order  to-day.  Some 
of  the  species,  both  living  and  fossil,  are  giants  among  the  rodents.  In 
the  tropical  OCTODONTID^  the  clavicles  are  complete,  and  the  molars  have 
internal  and  external  enamel  folds;  most  of  the  species  are  terrestrial. 
Cteuodactylus,  from  Africa.  Octodon,  from  South  America.  Myopotamus 
coypu,  of  South  America,  two  feet  long.  HYSTRICID^E  ;  porcupines  ;  stout 
rodents  with  spines,  molars  £.  The  species  are  grouped  in  two  divisions  : 
those  of  the  old  world  dwell  on  the  ground ;  those  of  the  new,  climb. 
Hystrix,  with  smooth  soles  and  incomplete  clavicles,  includes  the  old- 
world  porcupines ;  Erethyzon  and  Synetheres,  with  tuberculate  soles  and 
complete  clavicles,  those  of  the  new.  The  latter  genus  has  a  prehensile 
tail.  The  ChiNCHiLLiD^E,  with  very  soft  fur,  complete  clavicles ;  toes  five 
or  four  in  front,  four  or  three  behind ;  the  molars  with  simple  compressed 
transverse  lamellee,  are  confined  to  South  America.  Chinchilla.  Lago- 
stomns  includes  the  burrowing  vizcacha.  Megamys,  of  the  miocene  of 
Argentina,  was  as  large  as,  an  ox,  the  largest  known  rodent.  Somewhat 
closely  allied  to  them  was  the  fossil  Castoroides  of  N.  Y.  and  Ohio  pleisto- 
cene, and  the  Amblyrhiza  from  the  pleistocene  of  the  West  Indies.  The 
CAVIID^E,  also  South  American,  have  hoof-like  claws  on  the  four  toes  of 
the  fore  feet  and  the  three  of  the  hind ;  incomplete  clavicles,  and  the 
molars  |-  and  rootless.  Cai'ia  contains  the  guinea-pigs;  Hydrochcerus,  the 
capybara,  the  largest  existing  rodent.  Both  these  and  other  genera  date 
from  the  miocene.  The  agutis  (Dasyprocta)  and  the  paca  {C&logcnys) 
are  South  American  forms  with  hoof-like  claws  and  semi-rooted  molars 
which  form  the  family  DASYPROCTID^E. 


SUB-ORDER  4.     LAGOMORPHA  (DUPLICIDENTATA). 

Infraorbital  canal  small ;  dentition  i  | ,  c  g,  p  \  to  f ,  m  §  ;  the  enamel 
of  the  upper  incisors  extending  on  to  the  sides  ;  molars  high,  prismatic, 
without  roots  ;  tibia  and  fibula  distinct. 

The  lagomorphs  are  readily  distinguished  by  the  two  pairs  of  incisors 
in  the  upper  jaw.  The  seat  of  the  sub-order  is  in  the  northern  hem- 
isphere, but  they  extend  into  South  America  as  well.  The  LEPORID,E, 
with  the  premolars  f ,  long  ears  and  incomplete  clavicle,  includes  the  hares, 
and  rabbits.  Lepus,  the  principal  genus,  appears  in  the  miocene  of  Oregon. 
About  20  living  species.  In  the  LAGOMYID^:  the  premolars  are  \  or  f ,  the 
ears  short,  and  the  clavicle  complete.  The  picas  (Lagomys}  inhabit  high 
altitudes,  one  species  occurring  in  the  Rocky  Mountains.  The  genus 
appears  in  the  miocene  of  Bavaria. 


ORDER   V.     UNGULATA. 

Placental  mammals  with   heterodont,  diphyodont  dentition  ; 
molars  with  broad  tuberculate  or  ridged  crowns  ;  clavicles  almost 


392 


CLASSIFICATION  OF   VERTEBRATES. 


always  lacking,  digits  with  broad,  blunt  nails  or  more  usually 
with  hoofs  ;  digits  ranging  from  five  to  one,  radius  and  ulna  free 
or  united  ;  scaphoid  and  lunar  bones  (p.  177)  of  carpus  always 
free. 

If  only  the  living  forms  were  considered  the  characters  of 
the  ungulates  and  the  sub-divisions  of  the  order  could  be  easily 
given,  but  the  fossil  forms,  which  are  especially  well  developed 
in  our  western  states,  have  introduced  so  many  annectent 
groups  that  boundary  lines  tend  to  disappear,  while  to  almost 
every  character  exceptions  occur.  Were  the  recent  forms  alone 

considered  the  order 
would  contain  only  the 
artiodactyls  and  perisso- 
dactyls  of  the  following 
pages,  but  the  extinct 
species  connect  these 
so  closely  with  the  ele- 
phants and  Hyrax  that 


B 


GDCDQ 

000S 


/ 

CDQQ 


FIG.  363.  Types  of  carpal  bones  ;  A,  in  series 
(taxeopodous)  ;  B,  interlocking  (diplarthrous). 
R,  radius  ;  £/,  ulna  ;  c,  carpales  ;  *',  intermedium  ; 
r,  radiale  ;  u,  ulnare  ;  1-5,  metacarpals. 


all  must  be  included  un- 
der a  common  heading. 
The  existing  forms  are 
all  terrestrial  and  with 
few  exceptions  herbivor- 
ous, none  being  dis- 
tinctly predaceous.  For 

convenience  all  of  these  forms  may  be  divided  into  the  true 
ungulates  (Ungulata  Vera  or  Diplarthra)  and  the  Subungulata, 
the  former  including  the  artiodactyls  and  the  perissodactyls,  the 
latter  the  other  sub-orders  :  Condylarthra,  Amblypoda,  Probos- 
cidia,  Toxodontia,  and  Hyracoida. 

In  the  Ungulata  Vera  the  feet  are  never  plantigrade  ;  the 
digits  never  exceed  four,  the  first  being  suppressed ;  the  molars 
are  quadritubercular.  The  mammae  are  abdominal  or  inguinal 
in  position,  and  are  usually  few  in  number.  The  placenta  is 
nondeciduate,  and  is  either  diffuse  or  cotyledonary. 

In  the  Subungulata  the  feet  are  frequently  five-toed,  and 
they  may  be  plantigrade,  and  the  bones  of  the  first  row  of  the 
carpus  and  tarsus  are  in  a  direct  row  with  those  of  the  second, 


MAMMALS.  393 

while  in  the  ungulata  vera  they  alternate.  The  subungulata 
also  present  considerable  differences  in  the  placental  arrange- 
ments, allusion  to  which  will  be  made  below. 

Professor  Cope,  utilizing  the  characters  presented  by  the 
carpal  and  tarsal  bones,  has  proposed  to  divide  the  ungulates 
into  five  divisions,  —  Taxeopoda,  Toxodontia,  Proboscidia,  Am- 
blypoda  and  Diplarthra,  —  his  Taxeopoda,  including  not  only 
forms  usually  recognized  as  ungulates,  but  the  primates  as  well. 

SUB-ORDER  i.     CONDYLARTHRA  (MESODACTYLA). 

Extinct  ungulates  with  five-toed,  plantigrade  feet ;  carpalia  in  straight 
rows,  not  alternating;  femur  with  third  trochanter,  molars  bunodont. 

The  condylarthra  are  the  most  primitive  of  ungulates.    From  them  have 
doubtless  descended  the  ungulata  vera,  and,  if  the  views  of  Cope  be  correct, 
the  carnivores  and  primates  as  well.    The  group 
appears  in  the  lowest  eocene,  and  is  especially 
well    developed    in  the  lower  tertiaries   of  the 
western  U.  S.     Four  families,  PERIPTYCHID.E, 

PHENACODIDjE,  MENISCOTHERIID^,  and  PLEUR- 

ASPIDOTHERIID^E   recognized,    the  latter   from 
the  eocene  of  France. 


SUB-ORDER  2.     PERISSODACTYLA 
(SOLIDUNGULA). 

Unguligrade  ungulates  with  the  middle  toe 
well  developed,  forming  the  axis  of  the  foot, 
carpals  alternating ;  astragalus  with  pulley-like 
surface  for  tibial  articulation  ;  placenta  diffuse. 

The   perissodactyls,  which  walk  upon  the 
very  tips  of  the  toes,  have  the  feet,  as  a  rule,  with 
the  toes  three  or  four  in  front  and  three  behind  ; 
but  frequently  only  the  third  toe  on  either  foot 
comes  to  complete  development,  the  others  be- 
ing very  rudimentary.     The  dentition  is  usually 
complete,  the  molars  being  lophodont  or  rarely 
bunodont,  while  the  premolars  tend  to  resemble 
the  molars.     The  femur  (except  in  Chalicothe-          FIG.  364.     Fore  foot  of 
riuiri)  has  a  third  trochanter,  and  the  fibula  does     two-horned  rhinoceros,  Ate- 
not  usually  reach  the  calcaneum.     The  stomach     lodus  bicornis. 
is  simple  ;  the  intestine  has  large  caecum,  and  a 

gall  bladder  never  occurs.  The  mammae,  few  in  number,  are  inguinal  in  po- 
sition. The  living  perissodactyls  present  three  very  distinct  types, —  horses, 
tapirs,  and  rhinoceroses,  —  but  in  the  tertiary  period  many  other  forms  occurred 


394 


CLASSIFICATION  OF   VERTEBRA7ES. 


which  intergrade  between  these,  largely  obliterating  the  distinctions.  In  no 
group  of  mammals  have  the  lines  of  descent  been  worked  out  more  com- 
pletely than  here. 


FIG.  365.     A,  right  fore  foot  of  horse,  from  Huxley,     i,  radius  ;  3,  scaphoid ; 

4,  lunare ;    5,   cuneiforme ;    6,  pisiforme ;    7,    magnum;    8,    unciforme;    9,   meta- 
carpal  III ;  10,  splint  bone  (metacarpal  IV)  ;  1 1,  14,  sesamoid  bones  ;  12,  proximal 
phalanx  (fetter  bone);   13,  middle  phalanx  (coronary  bone)  ;   15,  distal  phalanx 
(coffin  bone).    B,  left  hind  foot,     i,  tibia  ;  2,  calcaneum  ;  3,  astragalus  ;  4,  cuboid ; 

5,  navicular  or  scaphoid  ;  6,  ectocuneiforme ;  7,  metatarsal  Til  (cannon  bone)  ;  8, 
metatarsal  IV  (splint  bone)  ;  9,  u,  12,  phalanges  (see  fig.  A)  ;   10,  14,  sesamoids. 


MAMMALS. 


395 


In  the  horses  (EguiD^:)  the  dentition  is  z f ,  c  \,  p  f  to  |,  m\-,  there  are 
from  four  to  one  toes  on  the  fore  feet,  three  to  one  on  the  hind  feet.  In 
Hyracotherinm  (Eo/iippiis)  the  toes  are  four  and.  three  on  the  fore  and  hind 
feet  respectively  ;  eocene.  P  alee  other  turn,  eocene  and  miocene  of  both  hemi- 
spheres, with  three  toes.  Mesohippus,  miocene.  In  Hipparion  and  Protohip- 
pus  toes  2  and  4  reduced  so  as  not  to  reach  the  ground,  but  furnished  with 
hoofs  ;  pliocene.  Equus,  the  existing  horses  and  asses,  has  toes  2  and  4  re- 
duced to  metacarpal  splint  bones  without  phalanges.  The  genus  appears  in 
the  Indian  miocene  and  a  little  later  in  North  America.  The  existing  species, 
including  the  asses  and  zebras,  all  belong  to  the  old  world.  The  PROTO- 
THERIDJE,  with  tridactyl  feet  and  incisors  ^,  range  through  South  American 
tertiary,  as  do  the  MACRAUCHENIID^E  with  the  incisors  \ ,  and  with  no  dias- 
tema  in  the  jaws. 

TAPIRID/E,  with  four  toes  on  the  fore  feet,  three  on  the  hind  (Fig.  348), 
range  from  the  eocene  to  the  present  time.  Lophodon,  Isectolophus,  eocene. 


FiG.  366.     Sumatran  rhinoceros,  Ceratorhinus  snmatrensts,  from  Liitken. 

Tapirus  arises  in  the  pliocene  of  Europe,  from  which  have  differentiated  the 
tapirs  of  India  and  tropical  America,  the  genus  dying  out  in  Europe  in  the 
pliocene.  The  existing  species  are  of  middle  size,  and  live  usually  in  woods 
or  swampy  places.  The  RHINOCERID/E,  or  rhinoceroses,  have  three  or  four 
toes  on  the  fore  feet,  the  hind  feet  tridactyl,  the  teeth  i  f  to  §,  c  \  or  g,  p 
|-  to  §,  m  f .  Some  of  the  extinct  forms  were  without  horns,  some  had  one 
horn  and  some  had  two,  either  one  behind  the  other,  as  in  the  existing  two- 
horned  species,  or  as  in  Dicer atherium^  from  the  miocene  of  Oregon,  the  two 
horns  were  placed  side  by  side.  HynicJnus  and  Hyracodon,  from  the  Ameri- 
can eocene  and  miocene,  were  hornless,  as  was  Aceratherium  of  the  oligocene 
and  miocene,  the  least  differentiated  of  the  true  rhinoceroses.  The  living 
species  are  distributed  in  Ceratorhinns,  two  horns,  from  Asia;  Atelodus* 
two  horns,  from  Africa ;  and  Rhinoceros,  a  single  horn. 


396  CLASSIFICATION  OF   VERTEBRATES. 

The  TiTANOTHERiiDyE  of  the  eocene  and  miocene  of  Europe  and  America 
were  mostly  large  animals  with  toes  four  and  three  on  the  fore  and  hind  feet 
respectively,  and  the  teeth  varying  between  i  §  to  f ,  c  },  p  f  to  £,  m  f . 
Palceosyops  (Lymnohyus).  Titanotheriuin  {Br  onto  t  her  ium)  has  a  pair  of 
large  bony  processes  on  the  snout,  probably  covered  with  horns.  One  spe- 
cies nearly  as  large  as  an  elephant. 

The  position  of  the  CHALICOTHERIID^E  is  uncertain.  In  the  teeth  it  is 
distinctly  perissodactyl,  but  its  three-toed  feet  were  plantigrade,  and  termi- 
nated with  long,  curved  claws.  The  family  ranges  from  the  eocene  to  the 
pliocene,  and  is  best  developed  in  Europe.  Moropns,  Macrotherium. 

SUB-ORDER  3.     ARTIODACTYLA. 

Unguligrade  or  digitigrade  ungulates  in  which  the  toes  are  symmetrically 
developed  about  an  axis  passing  between  the  third  and  fourth  digits.  Fre- 
quently a  reduction  from  the  full  dentition  of  44  teeth  ;  premolars  unlike  the 
molars,  the  former  with  one  lobe,  the  latter  with  two,  except  the  last,  which 
has  three  lobes  ;  femur  without  third  trochanter ;  fibula  articulating  with  the 
calcaneum.  Stomach  complex  ;  caecum  often  present,  large  and  convoluted  ; 
mammae  2  or  4,  inguinal ;  placenta  diffuse  or  cotyledonary. 

The  artiodactyls  are  mostly  large  animals,  distributed  all  over  the  earth, 
with  the  exception  of  the  Australian  region.  The  relations  of  the  axis  of  the 
foot  produce  the  well-known  cloven  hoof  so  characteristic  of  the  group,  while 
in  many  there  is  a  tendency  towards  the  loss  of  the  incisors  and  canines  in  the 
upper  jaw.  Another  common  feature  is  the  development  of  bony  horn  cores 
upon  the  frontal  bones.  The  recent  forms  are  frequently  sub-divided  into 
four  series,  Suina,  Tragulina,  Tylopoda,  and  Ruminantia  (Pecora,  or  Coty- 
lophora),  but  when  the  extinct  species  are  taken  into  account,  the  divisions 
break  down.  For  convenience  the  characters  of  these  groups  may  be  given 
here. 

Suina :  with  the  families  Hippopotamidae  and  Suidae,  with  bunoclont 
molars  and  distinct  or  but  partially  fused  third  and  fourth  metatarsals 
and  metacarpals ;  i.e.,  without  a  cannon  bone.  Tragulina:  with  the  family 
Tragulidae,  in  which  a  cannon  bone  usually  occurs  ;  and  the  stomach  is  three- 
chambered,  the  manyplies  being  absent ;  fibula  complete.  Tylopoda  :  includ- 
ing the  Camelidae :  with  only  digits  3  and  4  developed,  their  metapodials 
fused  above  ;  manyplies  absent,  red  blood  corpuscles  oval.  Ruminantia :  with 
the  families  Cervicornia  and  Cavicornia.  In  these  there  are  no  upper  inci- 
sors ;  a  cannon  bone  is  present,  the  stomach  four-chambered,  and  the  pla- 
centa cotyledonary.  The  processes  concerned  in  rumination  may  be  described 
here,  although  a  chewing  of  the  cud  occurs  also  in  the  camels.  When  feed- 
ing, the  food  as  swallowed  passes  into  the  paunch,  and  thence  to  the  honey- 
comb. In  both  of  these  it  is  softened,  and  then  is  regurgitated  into  the 
mouth,  and  masticated  by  the  teeth.  After  this  comminution  it  is  swallowed 
again,  but  at  this  time  it  passes  directly  to  the  third  stomach,  or  manyplies, 
and  thence  to  the  abomasum,  which  is  the  true  digestive  stomach. 

The  central  stem  of  the  artiodactyls  seems  to  be  the  PANTOLESTID^E  of 
the  American  eocene,  with  bunodont  molars,  and  probably  four-toed  feet. 


MAMMALS. 


397 


The  ANTHRACOTHERIID^:,  best  developed  in  the  European  upper  eocene, 


have  the  teeth  / 


\,  ///  f  ,  the  metapodials  distinct,  and  four  toes  on 


each  foot,  the  outer  ones  in  process  of  reduction.    Anthracotherinin,  eocene. 

Hyopotamus,  miocene  of  the  U.  S.  and  Europe.      The  SUID^J,  or  swine, 

apparently  derivatives  of  the  last  family,  appear  in  the  eocene  of  both  conti- 

nents, and  continue  to  the  present  time.     They  have  the  teeth  i  f  or  f  ,  c  \, 

P  \  to  f  ,  m  f  ,  the  molars  bunodont.     The  feet  are  four-toed,  rarely  three- 

toed,  toes   2  and  5  smaller  than  the  others,  and  the  metapodials  distinct. 

The  stomach  has  a  pouch  developed  near  the  cardiac  opening  ;  the  colon  is 

spirally  coiled,  and  a  caecum  is  present.     The  earlier  history  of  the  family  is 

less  certain  than  that  of  some  others,  and  some  of  the  earlier  genera  seem  to 

have  a  carnivorous  facies.     The  family  to-day  belongs  to  the  old  world,  only 

the  peccaries  (frequently  set  aside  as  a  distinct  family,  DICOTYLTD^E)  occurring 

in  the   western    hemisphere.      In 

Achanodon,  from  the  eocene,  there 

are  already  developed  the  tusk-like 

canines  so  characteristic  of  mod- 

ern swine  ;  in  Elotherium  they  are 

less  conspicuous,  while  in  Chocro- 

potamus    (eocene,    Europe)   and 

Leptochcerns     (miocene,     U.    S.) 

these  teeth  are  smaller.     The  pec- 

caries   (Dicotyles}   appear   in  the 

American  pleistocene,  and  two  or 

three   species   persist    in   warmer 

America  to-day.     They  have  the 

teeth  i  f  ,  c  j,  p  \  ,  m  f  ;  the  fifth 

toe  of  the  hind  feet  lacking,  and 

the  stomach  more  complex  than 

in  the  typical  swines.     The  spe- 

cies are  gregarious  and  omnivor- 

ous.     The   allied   Platygonus   is 

pliocene.      In  the  pigs  proper  — 

Sns,  Babirusa.  Phacochoeriis  —  the 

canines  are  greatly  developed  and  triangular  in  section,  and  a  large  diastema 

exists  between  these  and  the  premolars.    All  are  old-world  forms,  and  are  dis- 

tinguished by  the  dentition  :  Sus,  /-|,  c  \,  p  |-,  m  ^  ;  Babirusa,  i  \,c\,p\*  M  f  ; 

Phacockcerus*  i\,  c\,P  f,  m  \-    The  true  swine,  Sus,  appear  in  the  pliocene 

and  continue  as  our  domestic  hogs,  descended  from  the  wild  boar  and  other 

Asiatic  species.    The  single  species  of  Babirusa  (Porcits}  of  the  Malay  Islands 

is  remarkable  in  that  the  upper  canines  of  the  male  grow  upward  through  the 

skin  of  the    snout.      The  wart-hogs  of  Africa  (Phacockcerus]  receive  their 

common  name  from  the  projections  on  the  face.     In  the  adults  many  of  the 

teeth  are  lost,  but  the  canines  form  enormous    tusks,  both    pairs    curving 

upwards  and  outwards. 

The  HIPPOPOTAMID^E  are  large,  amphibious,  bunodont  forms,  with  teeth 
/  I  to  f,  c  \,  p  |,  m  f  ,  th^lower  incisors  very  long  and  rootless.  The  metapo- 
dials are  distinct,  the  feet  four-toed,  the  lateral  toes  being  nearly  equal  to  the 


FIG.  367.  Stomach  of  sheep,  after  Carus^ 
and  Otto  (Oppel).  a,  abomasum  ;  o,  oma- 
sum; re,  reticulum;  ru,  rumen. 


398  CLASSIFICATION  OF   VERTEBRATES. 

others  ;  the  digits  bear  nail-like  hoofs.  Restricted  to  the  eastern  hemisphere, 
where  they  occur  fossil  in  Europe  and  Asia  since  the  pliocene,  the  living 
species  are  all  African.  Three  genera  are  distinguished  by  the  number  of 
lower  incisors :  Hippopotamus  with  six,  Tetraprotodon  with  four,  and  Chce- 
ropus  with  two.  Hippopotamus  dates  back  to  the  pliocene,  Chceropns  is 
living;  Tetraprotodon  is  known  only  from  the  African  pleistocene.  The 
common  species,  H.  amphibius,  has  an  enormous  three-chambered  stomach, 
eleven  feet  in  axial  length.  These  animals  are  gregarious  and  herbivorous. 

The  OREODONTID.E  lived  from  the  eocene  to  the  pliocene  of  North  Amer- 
ica. They  had  the  teeth  i  f ,  c  \,  p  \,  m  f ,  and  the  feet  four-toed  ;  and  in  Pro- 
toreodon,  from  the  eocene,  the  fore  toes  were  five  in  number.  Agriochcerus  and 
Oreodon  miocene,  Merychius  (Ticholeptus)  miocene  and  pliocene.  The  CA- 
MELID^E  (Tylopoda),  which  appear  in  the  eocene,  may  have  descended  from 
either  the  pantolestidas  or  the  oreodontidae.  They  have  the  teeth  /  f  to  \,  c  ^-, 
p  4  to  f ,  m  f ,  the  molars  selenodont,  with  a  diastema  between  the  premolars 
and  canines ;  the  feet  four-  or  two-toed,  the  lateral  toes  completely  lost  in 
the  more  recent  species  ;  and  in  all  except  the  older  forms  the  metapodials 
fused  to  a  cannon  bone.  The  stomach  lacks  the  manyplies,  while  rumen 
and  honeycomb  are  sub-divided  into  numerous  small  cavities  on  the  walls. 
The  placenta  is  diffuse.  The  living  genera,  Camelus,  which  inhabits  Asia 
and  Africa,  and  Aitchenia  of  South  America,  appear  in  the  pliocene.  They 
differ  in  the  premolars,  these  being  f  and  \  respectively.  The  camels  are 
two  in  number,  the  single-humped  dromedary,  C.  dromedarius,  and  the  two- 
humped  bactrian,  C.  bactrianus.  Auchenia  contains  four  species,  the  llama, 
alpaca,  vifuna,  and  guanaco.  Among  the  extinct  genera  are  Leptotragnlus, 
eocene ;  PoebrotJierium  and  Protolabis,  miocene ;  Procamelus,  EscJiatius, 
and  Pliauchenia,  pliocene.  The  ANOPLOTHERIID^E  of  the  European  eocene 
and  pliocene  are  noticeable  from  the  fact  that  it  was  in  this  group  that 
Cuvier  made  many  restorations.  Anoplotherium,  Dichobune,  Ccenotlierium, 
Xiphodon. 

The  TRAGULID^E,  or  chevrotains,  have  the  teeth  i  §,  c  \,p  f  or  |,  ;// 1 ; 
fibula  complete,  usually  a  cannon  bone  ;  feet  four-toed  ;  stomach  three-cham- 
bered ;  placenta  diffuse.  These  forms  have  been  closely  associated  with  the 
musk  deer.  Tragulus  of  Asia  contains  the  smallest  existing  ungulates. 
Dorcatherium  (Hyomoschns}  from  Africa.  Leptomeryx,  American  miocene. 

The  CERVICORNIA,  in  which  the  teeth  are  z'f  to  f ,  c  \,  p  f  to  f  m,  f ,  the 
upper  canine  being  sometimes  very  large,  sometimes  small  or  absent,  the 
molars  selenodont,  are  as  a  rule  characterized  by  the  development  of  horns 
or  antlers  upon  the  frontal  bones  of  the  male,  although  they  are  occasionally 
absent,  or.  as  in  the  reindeer,  they  may  appear  in  both  sexes.  These  horns 
consist  of  a  bony  outgrowth  from  the  frontals  ;  and  at  first  this  is  covered  with 
skin,  which  may  persist  through  life,  as  in  the  giraffe,  or,  more  usually,  is 
soon  worn  off,  leaving  the  bone  alone.  Each  year  this  horn  is  shed,  and  a 
new  antler  takes  its  place,  the  later  one  displaying  a  greater  number  of 
branches  or  '  tines,1  so  that  these  become  an  index  of  age.  Metapodials 
3  and  4  usually  form  a  cannon  bone ;  the  lateral  metapodials  are  reduced, 
and  the  toes  do  not  reach  the  ground.  The  stomach  and  placenta  are  of  the 
ruminant  type  (p.  396).  The  species  are  very  numerous,  but  none  occur  in 


MAMMALS.  399 

Australian  regions,  and  only  the  giraffes  in  Africa.  The  family  is  not  known 
previous  to  the  lower  miocene  of  Europe,  the  earliest  forms  showing  relation- 
ships to  the  tragulines  and  antilopes.  The  musk  deer  (Moshus,  Hydropotes} 
are  without  horns,  the  upper  canine  is  long  and  projecting,  and  the  male  has 
a  '  musk  gland '  situated  beneath  the  skin  of  the  abdomen.  The  species  be- 
long to  Central  Asia.  The  living  muntjacs  are  also  Asiatic,  but  their  ances- 
tors appeared  in  the  miocene  of  both  hemispheres.  Cope  and  Schlosser 
regard  the  group  as  the  ancestors  of  the  true  deer  and  of  the  antilopes  as 
well.  Cervulus,  muntjacs  of  Asia.  Blastomeryx,  American  miocene.  Coso- 
ryx,  American  pliocene.  The  true  deer  (Cervus)  are  characterized  by  the 
presence  of  horns.  They  are  usually  sub-divided  into  many  subgenera,  Axis, 
Cariacus,  Elaphns,  etc.,  upon  characters  of  minor  importance;  more  dis- 
tinct are  the  moose  (Alces)  and  the 
reindeer  or  caribou  (Rangifer). 
The  deer  are  largely  inhabitants  of 
the  northern  hemisphere.  Con- 
siderably different  is  Protoceras 
from  the  American  miocene,  in 
which  there  were  rudimentary  horn 
cores  on  the  frontals  and  parietals, 
and  vertical  bony  plates  on  the 
maxillae,  while  the  canines  recall 
those  of  Tragulus.  The  giraffes  FlG<  s68>  Successive  antlers  of  the  red 
(Giraffa  or  Camelopardalis,  often  deer  (Cervus  elaphus},  after  Gaudry. 
grouped  as  a  family,  Devexa)  have 

long  legs,  and  short  non-deciduous  horns.  Allied  to  these  in  structure,  but 
lacking  the  characteristic  long  neck,  occur  in  the  European  and  Asiatic 
miocene  Helladotherinm,  Saniotherium.  Sivatherium,  with  a  single  large 
species  from  the  Indian  miocene,  combines  giraffe  and  antilope  characters. 

In  the  family  CAVICORNIA  the  horns  are  almost  always  borne  by  both 
sexes,  and,  unlike  those  of  the  cervicornia,  have  the  bony  horn  cores  covered 
with  true  or  epidermal  horn.  With  rare  exceptions  the  horns  are  never  shed  ; 
the  teeth  are  /  f ,  c  f ,/  f ,  m  f  ;  the  median  metapodials  form  a  cannon  bone  ; 
the  laterals  are  greatly  reduced  or  entirely  absent.  The  family,  which  is  rich- 
est in  species  of  any  of  the  ungulates,  appears  to  have  descended  from  the 
muntjacs  through  the  antilopes.  The  species  are  usually  arranged  in  antilo- 
pine,  ovine,  and  bovine  series,  the  three  being  distinct  in  the  pliocene.  The 
antilopes,  which  appear  in  the  miocene,  have  the  round  or  triangular  horns 
close  behind  the  eyes,  the  middle  incisors  largest.  Antilope,  India;  Saiga, 
with  large  inflated  nose,  Asiatic ;  Gazella,  Asiatic,  the  springbok  (G.  eu- 
chore),  African  ;  Oryx,  thegemsboks  ;  Catoblephas,  gnus  ;  Rupricapra  tragns, 
the  chamois  ;  closely  allied  is  the  Rocky  Mountain  goat,  Haploceras  montanus. 
Antilocapra  americana,  the  prong-horn  of  western  U.  S.,  is  remarkable  for 
its  deciduous  horns.  In  all  over  a  hundred  living  antilopes  are  known. 
Among  the  fossil  genera  are  Cosoryx,  Antidorcas,  Tragelaphus,  etc.  The 
ovine  series,  which  includes  the  sheep  and  goats,  has  the  laterally  com- 
pressed, transversely  ribbed  horns  with  hollow  cores  borne  close  behind  the 
eyes,  and  the  incisors  similar.  None  are  known  before  the  pliocene.  Capra, 


400 


CLASSIFICATION  OF   VERTEBRATES. 


ibex,  and  goats,  the  domestic  goats  supposed  to  descend  from  C.  agagrus  of 
the  eastern  Mediterranean  region.     Ovis,  the  sheep,  with  several  European 

species  and  one  American,  the  big-horn, 
O.  inontana.  Ovibos  inoschatus,  the 
'  musk  ox,1  a  goat  rather  than  an 
ox,  is  confined  at  present  to  Arctic 
America.  In  pleistocene  times  it  ranged 
over  Siberia  and  Europe,  south  to 
France  and  England.  In  the  bovine 
series  the  horns  are  strong,  some  dis- 
tance behind  the  eyes,  often  on  the  pos- 
terior angle  of  the  head;  the  frontals 
large,  theparietals  small.  Fossil  species 
first  appear  in  the  miocene  of  India, 
later,  in  the  pliocene  of  Europe  and 
America.  Bnbalus,  the  buffalo  of  India 
and  Africa ;  Bibos,  the  domesticated 
Indian  cattle,  and  the  yak  and  ban- 
teng  ;  Bison,  the  aurochs  of  Europe  and 
the  '  buffalo  '  of  America,  both  near  ex- 
tinction. To  Bos  belong  the  domestic 
cattle,  and  the  now  extinct  '  ur '  of 


FIG.  369.     Prong-horn  antilope. 
Antilocapra  americana. 


Europe,  which  possibly  lived  as  late  as  the  composition  of  the  Nibilungen 
tales.1  There  is  evidence  to  show  that  our  domestic  cattle  are  descended 
from  several  distinct  races. 

SUB-ORDER  4.     AMBLYPODA. 

Large,  extinct,  semiplantigrade  ungulates,  the  pentadactyl  feet  having 
the  distal  phalanges  surrounded  by  hoofs  ;  carpals  alternating,  molars  lopho- 
dont,  brain  very  small. 

These  forms,  which  begin  in  the  lowest  eocene,  are  regarded  by  Cope  as 
the  ancestors  of  both  artiodactyls  and  perissodactyls.  They  also  show  pro- 
boscidian affinities.  Pantolambda.  CorypJwdon,  with  complete  dentition, 
feet  digitigrade  in  front,  plantigrade  behind,  ranged  through  the  lower  and 
middle  eocene.  The  species  of  Uintatheriuni  (Dinoceras)  were  elephantine 
in  size,  and  bore  on  the  head  three  pairs  of  large  bony  processes  which  may 
or  may  not  have  borne  horns. 

SUB-ORDER  5.     PROBOSCIDEA. 

Pentadactyle  ungulates  with  long  proboscis  (nose),  skull  increased  in 
size  by  vacuities  in  the  bone ;  incisors2  never  exceeding  a  pair  in  either  jaw, 
frequently  the  upper  or  the  lower  lacking;  no  canines,  molars  lophodont ;  no 
clavicles  ;  radius  and  ulna,  tibia  and  fibula  distinct ;  stomach  simple,  caecum 

1  It  is  often  suggested  that  the  cattle  of  Chillingham  Park,  England,  are  descendants. 
of  these  wild  cattle. 

2  Frequently  spoken  of   as  canines ;  however,  they  arise  in  the  premaxilla,  and  only- 
later  do  the  roots  extend  back  into  the  maxillae. 


MAMMALS.  401 

large,  uterus  bicornuate  ;  placenta  zonary,  non-deciduate  ;  mammae  pectoral, 
two  in  number. 

The  elephants  and  their  extinct  relatives  form  a  most  interesting  group, 
highly  differentiated  in  some  respects,  more  generalized  in  others.  Most 
strange  is  the  proboscis  with  the  nostrils  at  the  tip.  while  the  incisors  which 
furnish  most  of  the  ivory  of  commerce  are  almost  solely  dentine,  the  enamel 
covering  the  tip  being  worn  off  at  an  early  date  (elephants) ,  or  forming  a 
band  on  the  outer  side  (Mastodon). 

The  DINOTHERID.E,  with  the  single  genus  Dinotherium,  occur  in  the 
miocene  and  pliocene  of  Europe  and  India.  In  this  only  the  lower  incisors 
are  present,  and  these  grow  downwards,  the  symphysis  of  the  jaw  being  bent 
so  that  the  teeth  form  downward-extending  tusks.  The  succession  of  molar 
teeth  was  normal.  The  animals  were  about  as  large  as  an  elephant. 

In  the  elephants  and  their  near  relatives  (ELEPHANTID/E)  the  upper 
incisors  form  tusks  of  varying  size,  the  lower  are  smaller  or  wanting.  The 
molars  bear  more  than  two  transverse  ridges,  and  are  subject  to  horizontal 


FIG.  370.  Evolution  of  the  teeth  of  elephants  after  Flower.  A,  Mastodon  ; 
B,  Stegodon  ;  C,  Elephas  (Loxodon}  africanus.  Dotted,  cement ;  obliquely  lined, 
dentine  ;  heavy  black  line,  enamel. 

replacement.  Owing  to  the  shortness  of  the  jaw,  and  the  large  size  of  the 
molars,  not  more  than  two  can  be  in  use  at  the  same  time.  As  the  more 
anterior  of  these  becomes  worn,  it  drops  out  in  front,  while  another  takes,  its 
place  from  behind.  In  some  Mastodons  transitional  types  of  succession  occur. 
Mastodon  (with  several  sub-generic  divisions)  extended  from  the  upper  mi- 
ocene through  the  pliocene,  and  may  have  been  contemporaneous  with  man. 
Its  teeth,  characterized  by  from  three  to  six  transverse  ridges,  or  ridges  broken 
into  tubercles,  are  common  in  the  northern  hemisphere,  Africa,  and  South 
America.  The  skeletons  are  less  perfectly  preserved.  Stegodon,  from  the 
later  tertiaries  of  India,  etc.  Elephas  (including  Loxodori)  embraces  the 
existing  elephants  as  well  as  the  extinct  mammoths.  In  these  the  valleys 
between  the  transverse  enamel  folds  are  filled  with  cement.  The  living 
species,  E.  indicus  and  E.  africanus  occur  in  India  and  Africa,  respectively. 
The  genus  appears  in  the  miocene,  the  mammoth,  E.  primigenius  of  the 
pleistocene,  which  became  extinct  after  the  appearance  of  man,  being  the  best 
known.  The  discovery  in  1799  of  frozen  mammoth  bodies  near  the  Lena 
delta  should  be  mentioned.  The  body  was  covered  with  close,  woolly  hair, 
while  a  mane  on  the  neck  and  the  hairs  on  the  head  were  three  feet  in  length. 


402  CLASSIFICATION  OF   VERTEBRATES. 


SUB-ORDER  6.     TOXODONTIA. 

Extinct  ungulates  with  tri-  or  pentadactyl  semiplantigrade  feet ;  alternat- 
ing carpalia ;  femur  without  third  trochanter ;  fibula  articulating  with  the 
calcaneum ;  third  toe  the  larger.  Canines  reduced,  sometimes  to  a  great 
extent. 


FlG.  371.     Skull  of  Typotherium  cristatum. 

The  Toxodons  and  their  allies  occur  in  the  tertiary  of  southern  South 
America,  and  are  as  yet  imperfectly  known.  They  exhibit  a  strange  associa- 
tion of  resemblances  to  perissodactyls,  Hyrax,  elephants,  and  rodents.  Tox- 
odon,  which  persisted  from  the  older  miocene  to  the  pleistocene,  was  about 
the  size  of  a  rhinoceros.  Nesodon,  from  the  eocene.  Sometimes  Typotherium 
and  its  allies  from  the  same  beds  are  separated  as  a  distinct  sub-order  Typo- 
theria. 

SUB-ORDER  7.     HYRACOIDEA. 

Small  plantigrade  ungulates  with  tridactyl  hind  feet,  fore  feet  four-toed, 
the  carpalia  in  series,  the  digits  with  nails;  femur  with  third  trochanter; 
teeth  i  ^,  c  §,  p  £,  m  f  ;  placenta  zonary.  Only  a  single  genus  Hyrax  (with 
several  sub-divisions)  known,  and  this  only  in  the  existing  fauna.  The  few 
species  described  come  from  Syria,  Arabia,  and  Africa.  They  live  in  holes 
in  the  rocks,  or  in  hollow  trees,  and  some  of  the  African  species  are  arboreal. 
One  species,  H.  syriacus,  is  supposed  to  be  the  '  coney '  of  the  Bible. 


SUB-ORDER  8.     TILLODONTA    (INCLUDING   T^ENIODONTA). 

Extinct  plantigrade  animals  with  pentadactyl  feet ;  teeth  zf  to  f,  c  {, 
p  |  to  §,  ///  f ,  the  upper  molars  with  three  cones,  the  lower  lophodont. 

These  animals  of  large  or  moderate  size  recall  in  some  respects  the  car- 


MAMMALS.  403 

nivores,  and  in  some  the  rodents.  They  belong  largely  to  the  eocene,  and 
the  United  States  has  furnished  the  greater  number  of  specimens,  Europe 
having  but  few.  Cope  has  united  these  forms  with  the  insectivores  and  cre- 
odonts  into  an  order  Bunotheria.  Esthonyx*  eocene,  New  Mexico.  Tilfo* 
therium,  eocene,  Wyoming.  Calamodon  {Styliiiodoii} .  Stagodon,  cretaceous. 

ORDER   VI.     SIRENIA. 

Thick  skinned,  naked  or  sparsely  haired,  aquatic,  placental 
mammals,  with  monophyodont  teeth  ;  with  fin-like  fore  limbs  ; 
hind  limbs  lacking  ;  a  horizontal  caudal  fin  ;  a  movable  elbow 
joint ;  small  brain  with  few  and  shallow  convolutions  ;  two  pectoral 
mammae. 

The  sirenia  contains  a  few  aquatic  mammals,  which  externally 
resemble  the  whales  in  their  fusiform  bodies,  flipper-like  fore 
limbs,  absence  of  hind  limbs,  and  flattened  caudal  fin.  In  more 
important  features  they  are  greatly  different,  and  nothing  that 
is  known  of  development  or  geological  history  points  to  their 
having  descended  from  a  common  stock.  They  have  the  nostrils 
separate,  and  opening  forward,  small  eyes  with  well-developed 
nictitating  membrane,  no  conch  to  the  ear,  no  dorsal  fin.  The 
paddle-shaped  fore  limbs  have  the  digits  enveloped  in  the  com- 
mon integument,  and  only  occasionally  are  nails  present.  The 
bones  are  very  dense,  and  the  long  bones  are  without  central 
cavities.  Only  occasionally  are  any  of  the  cervical  vertebrae 
anchylosed,  and  in  the  recent  forms  no  vertebrae  unite  to  form 
a  sacrum.  The  anterior  caudal  vertebrae  bear  chevron  bones. 
In  the  skull  the  chief  features  are  the  great  development  of  the 
zygomatic  arch,  the  reduced  nasals,  the  downward  flexure  of  the 
jaws  in  front,  and  the  lower  jaw  with  an  ascending  ram  us.  In 
the  fore  arm  the  radius  and  ulna  are  usually  anchylosed  at  their 
extremities  ;  the  digits  are  always  five,  and  there  is  no  such  in- 
crease in  the  number  of  phalanges  as  occurs  in  the  cetacea.  No 
clavicles  are  developed.  The  pelvis  is  represented  by  a  pair  of 
bones  lying  at  some  distance  from  the  vertebral  column. 

Incisors  and  molars  alone  are  present  in  the  recent  forms, 
and  in  one  genus  (Rhytina)  no  teeth  occur,  at  least  in  the 
adult.  Many  fossil  species  had  a  more  heterodont  dentition, 
and  in  Halitherium  there  was  a  milk  dentition  not  known  in 


404 


CLASSIFICATION  OF   VERTEBRATES. 


FIG.  372.     Restoration  of  Halitherium  schinzii,  after  Miss  Woodward. 


FIG.  373.     Manatee,  Manatus  americanus,  after  Elliott. 


MAMMALS.  405 

recent  forms.  In  place  of  teeth,  horny  plates  are  developed  in 
the  palatal  region  and  at  the  front  of  the  lower  jaw,  and  these 
are  masticatory  in  function.  The  stomach  is  divided  into  two 
principal  chambers,  and  these  in  turn  may  be  sacculated.  The 
diaphragm  is  oblique,  the  lungs  very  long,  and  the  heart  is  bifid 
at  the  tip,  the  two  ventricles  being  partially  separated  (Fig.  352). 
The  testes  are  abdominal  in  position  ;  the  uterus  is  two-horned. 
The  placentation  of  the  dugong  alone  is  known.  This  form  has 
a  non-deciduate  zonary  placenta. 

The  living  species  of  sea-cow  are  few.  All  are  littoral  in 
their  habits,  but  never  leave  the  water.  They  feed  upon  sea- 
weed, or  upon  the  grasses  growing  in  the  rivers.  They  are  per- 
fectly harmless,  although  they  attain  considerable  size.  These 
animals  may  afford  the  grain  of  truth  in  the  mermaid  myth. 

The  PRORASTOMID/E  (only  genus  Prorastonis  from  the  eocene  of 
Jamaica),  is  known  only  from  the  skull.  It  is  remarkable  in  having  a  com- 
plete dentition  :  z  f ,  c  \,  p  f ,  m  f .  The  MANATID.E  have  molars  8  to  10,  the 
first  single-rooted;  incisors  and  canines  never  functional.  Manatus  (Tri- 
chechus*)  includes  the  manatees  of  tropical  America  and  Africa.  Fossil  in 
pleistocene  of  South  Carolina.  They  have  but  six  cervical  vertebrae.  Here 
possibly  belong  the  tertiary  Manatherium  and  Ribodon.  HALICORID^E  with 
heterodont  molars,  and  either  with  tusk-like  incisors  in  the  upper  jaw  or  these 
lacking.  Halicore  includes  the  dugongs  of  the  Indian  Ocean.  Halitherium, 
from  the  miocene  and  pliocene  of  western  Europe,  gives  evidences  of  a  milk 
dentition.  Rhytoidus,  Felsinotherium,  from  the  tertiary.  RHYTINDI/E,  with- 
out teeth.  The  only  known  species,  RJiytina  stelleri,  the  northern  sea-cow 
of  the  northern  Pacific,  was  exterminated  in  the  last  century. 

ORDER    VII.     CETACEA. 

Aquatic  mammals  without  distinct  neck  ;  fore  limbs  paddle- 
like  ;  hind  limbs  absent ;  usually  a  dorsal  fin  ;  caudal  fin  in  two 
lobes  or  <  flukes,'  nostrils  011  the  top  of  the  head  ;  teeth,  when 
present,  homodont  and  monophyodont  ;  no  elbow  joint ;  brain 
large,  cerebrum  complicately  convoluted  ;  placenta  non-deciduate, 
diffuse. 

The  whales,  like  the  sea-cows,  form  a  distinctly  circum- 
scribed group,  sharply  marked  off  from  all  others,  so  that  no 
clear  conclusions  can  be  reached  as  to  their  line  of  descent  from 
other  groups,  although  one  is  justified  in  believing  that  they 


406  CLASSIFICATION  OF   VERTEBRATES. 

have  come  from  some  more  normal  quadripedal  mammal.  More 
recently  the  view  has  been  gaining  ground  that  the  two  living 
divisions,  the  toothed  and  the  whalebone  whales,  may  have  had 
diverse  origins,  and  their  present  resemblance  may  be  due  to 
convergence  rather  than  to  community  of  descent.  The  best 
guesses  as  to  their  ancestors  would  trace  them  either  to  the 
carnivores  or  to  some  long-tailed  ungulate,  while  the  presence 
of  dermal  ossicles  in  one  species  of  Neomeris,  and  possibly  in 
Zeuglodon,  needs  to  be  taken  into  account. 

The  skin  is  smooth  and  naked,  without  hairs  ;  even  the 
bristles  around  the  mouth  may  disappear  in  the  adult.  There 
is  no  neck  ;  the  head  is  large,  and  may  form  one-third  of  the 
total  length.  The  eye  is  small,  and  without  nictitating  mem- 
brane ;  the  nostrils,  separate  or  with  a  common  crescentic 
opening,  are  on  the  top  of  the  head  ;  there  is  no  external  ear, 
the  small  meatus  opening  close  behind  the  eye.  Beneath  the 
skin  is  the  thick  layer  of  fat  or  *  blubber.'  The  bones  are 
light  and  spongy.  The  cervical  vertebrae  are  more  or  less 
completely  fused,  and  have  the  zygopophyses  poorly  developed. 
There  is  no  sacrum,  but  the  caudal  vertebrae  are  distinguished 
from  the  lumbar  by  the  presence  of  chevron  bones.  The 
sternum  is  very  variable,  and  is  reached  by  only  one  (mys- 
tacocetes),  or  a  few  ribs.  The  skull  consists  of  a  nearly  spherical 
cranium,  from  which  the  facial  part  projects  like  a  beak.  In 
the  cranium  the  roof  is  formed  by  the  supraoccipital  and  inter- 
parietal,  which  extend  forward  to  meet  the  frontals,  excluding 
the  parietals  from  the  middle  line.  The  frontals  are  expanded 
laterally  to  form  roofs  to  the  orbits.  The  maxillae  are  very 
large,  the  nasals  very  small.  The  lower  jaw  lacks  an  ascending 
ramus.  Clavicles  are  lacking,  and  there  is  no  elbow  joint. 
The  bones  of  the  wrist  and  hand  remain  almost  entirely  carti- 
laginous in  the  whalebone  whales.  The  digits  are  four  or  five, 
the  phalanges  of  the  second  and  third  being  increased  in  num- 
ber up  to  fourteen,  a  condition  recalling  the  ichthyosaurs  (p.  3 1 2). 
The  pelvis  is  represented  by  two  bones,  free  from  the  vertebral 
column,  which,  on  account  of  their  muscular  relations,  are 
regarded  as  ischia.  In  some  species  rudiments  of  the  skeleton 
of  hind  limbs  occur,  imbedded  in  the  flesh. 


MAMMALS. 


407 


The  teeth  in  existing  species  are  homodont  and  monophyo 
dont,  and  never  pierce  the  gums  in  the  mystacocetes.  In  the 
fossils  there  is  evidence  of  descent  from  heterodont  ancestors. 


Pmx 


Pmx 


FIG.  374.  Skull  of  foetal  cachelot  whale  {Physeter~},  from  Huxley.  AS, 
alisphenoid  ;  A u,  auditory  region;  BO,  basioccipital ;  BS,  basisphenoid  ;  £O,exoc- 
cipital;  Eth,  ethmoid;  Fr,  frontal;  Ju,  jugal  (displaced  in  the  side  view  from  its 
connection  with  the  squamosal);  MX,  maxillary;  N,  external  nares;  Pa,  parietal; 
PI,  palatine;  Pmx,  premaxillary;  SO,  supraoccipital;  Sy,  squamosal;  Vo,  vomer. 

The  number  varies  greatly.  The  stomach  (p.  367)  is  several 
chambered,  the  intestine  is  comparatively  simple,  a  caecum  being 
present  in  some  species.  The  liver  has  four  lobes,  and  no  gall 


408  CLASSIFICATION  OF   VERTEBRATES. 

bladder  is  present.  The  larynx  is  prolonged  so  that  it  enters 
the  choana,  a  condition  recalling  the  marsupials.  The  testes 
are  abdominal,  the  uterus  two-horned,  and  the  two  mammae  are 
in  grooves  near  the  vulva.  These  are  provided  with  constrictor 
muscles,  by  which  the  milk  is  forced  into  the  mouth  of  the 
young.  About  two  hundred  existing  species  are  known. 

The  cetacea  are  introduced  by  the  zeuglodons  in  the  eocene, 
the  other  groups  appearing  in  the  miocene. 

SUB-ORDER  i.     ARCH^EOCETI. 

External  nostrils  at  the  middle  of  the  muzzle  ;  nasals  long ;  temporal 
fossa  elongate  ;  ribs  bicipital ;  anterior  teeth  with  single  roots,  posterior  with 
two  roots,  the  free  edges  dentulate.  Breast  bone  of  several  sternebrae. 
Cervicle  vertebrae  free. 

Zeuglodon  {Basilosaurus}  the  only  genus  (with  several  subgenera)  comes 
from  the  eocene  of  our  southern  states,  Europe,  and  New  Zealand.  One 
species  was  60  feet  long. 

SUB-ORDER  2.     ODONTOCETI  (DELPHINOIDEA,   DENTICET^E). 

Skull  asymmetrical ;  external  nares  united  at  base  of  snout;  nasals  very 
small ;  temporal  fossa  short ;  teeth  present  in  both  jaws  or  only  in  lower, 
occasionally  reduced  to  a  single  pair.  Olfactory  organs  absent  or  rudimen- 
tary ;  anterior  ribs  bicipital,  others  with  only  a  tubercular  head  ;  sternum  of 
several  sternebrae. 

The  toothed  whales  are  all  carnivorous,  but  much  as  they  have  been 
pursued  by  man,  many  questions  concerning  the  features  presented  by  any 
species  at  different  ages  are  still  unsettled.  The  SQUALODONTTID^E,  with 
teeth  in  both  jaws,  and  these  differentiated  into  incisors,  canines,  and  two 
kinds  of  molars,  occur  in  the  miocene  and  pliocene.  In  some  respects  they 
seem  intermediate  between  the  zeuglodons  and  the  toothed  whales,  but 
their  skeleton  is  imperfectly  known.  The  PLATANISTID^:  include  the  fresh- 
water dolphins,  Platanista  and  Inia,  from  the  Ganges  and  Amazons  ;  allied 
forms  occur  in  the  miocene  and  pliocene  of  both  continents.  The  true  dol- 
phins (DELPHINID^:)  have  the  snout  elongate,  no  premaxillary  teeth,  the  other 
teeth  variable,  usually  conical,  and  with  a  single  root.  Teeth  are  numerous 
in  both  jaws  of  the  true  dolphins  and  porpoises  {Delphinus,  Turswp*,  Pho- 
c<zna,  Neomeris)  ;  are  fewer  in  the  black  fish  (Globiocepkaltis)  and  the  killer 
whales  (Orca\  these  latter  being  the  wolves  of  the  sea.  The  white  whale  of 
the  Arctic  seas  is  Delphinaptcrus,  which  in  all  points  of  structure  except  the 
numerous  teeth  is  closely  allied  to  the  narwal  (Monodori)^  in  which,  in  the  fe- 
male, all  the  teeth  are  functionless,  while  in  the  male  one  left  maxillary  tooth  is 
developed  into  an  enormous  spirally  twisted  tusk,  which  sticks  straight  out  from 
the  head.  Its  functions  are  unknown.  Several  of  these  genera,  the  narwal  in- 
cluded, occur  as  fossils  in  the  later  tertiary  and  pleistocene.  The  PHYSETER- 


MAMMALS. 


409 


includes  large  and  medium-sized  whales  with  toothless  upper  jaws. 
Physeter,  which  includes.the  sperm  whales,  dates  from  the  pliocene  of  western 
Europe  and  South  Carolina.  It  has  numerous  (20-25)  conical  teeth  in  the 
lower  jaw.  The  head  is  high,  truncate  in  front,  and  in  the  '  chair '  between 


FlG.  375.     Pigmy  whale,  Kogia  Jioweri,  from  Gill. 

the  cranium  and  snout  occurs  a  thick  waxy  oil,  the  spermaceti.  The  sperm 
whale  (P.  macrocephalus}  furnishes  the  ambergris  which  is  really  imperfectly 
digested  squid  become  concretionary  in  the  intestines.  ZipJiins  includes  the 
two-toothed  whales,  so  called  from  the  existence  of  but  a  pair  of  teeth  in  the 
lower  jaw.  Allied  are  Hyperoodon,  the  bottle-nosed  whales,  Mesoplodou,  and 
the  pigmy  whales,  Kogia. 

SUB-ORDER  3.     MYSTA- 
COCETI  (BAL^ENOIDEA). 

Skull  symmetrical ;  two 
nostrils  at  the  base  of 
the  snout ;  temporal  fossae 
short ;  functional  teeth  lack- 
ing, the  upper  jaw  bearing 
plates  of  baleen ;  most  of 
the  ribs  with  a  single  head  ; 
sternum  broad,  in  one  piece, 
and  articulating  with  the 
first  pair  of  ribs  alone. 

The  whalebone  whales 
have  teeth  in  both  jaws  in 
the  foetal  stage,  but  these 
never  pierce  the  gums,  and  FlG  ^  Cross.section  through  the  jaws  of  a 
are  absorbed  before  birth.  whalebone  whale>  after  Deiage.  Bones  black ;  w, 
After  birth  a  series  of  flat-  whalebone?  hanging  down  into  the  cavity  of  the 
tened  plates  of  horny  mate-  mouth>  m  .  f>  ethmoid  ;  mx,  maxillary  ;  /,  parietals  ; 
rial  make  their  appearance  ^  vomer. 
on  either  side  of  the  upper 

jaw.  These  plates,  of  which  there  are  several  hundred  pairs,  are  triangular 
in  shape,  with  the  inner  edges  fringed  out  into  hair-like  fibres,  the  plates  being 
at  righ't  angles  to  the  axis  of  the  body,  and  the  whole  forms  a  very  efficient 
straining-apparatus,  by  means  of  which  the  whales  separate  the  small  animals 
on  which  they  feed  from  the  surrounding  water.  Morphologically  the  baleen. 


4IO  CLASSIFICATION  OF   VERTEBRATES. 

consists  of  large  numbers  of  cornified  papillae  arising   from  the  ectoderm 
lining  the  cavity  of  the  mouth. 

In  the  BAL^ENOPTERID^:  the  head  is  less  than  a  fourth  of  the  total  body 
length,  the  ventral  side  of  the  body  is  usually  marked  by  longitudinal  grooves, 
a.  dorsal  fin  (a  tegumentary  fold  without  skeleton)  is  present,  the  hands  have 
four  digits,  and  the  cervical  vertebrae  have  their  large  centra  free.  Baltcnop- 
iera,  with  the  head  small  and  flat,  and  the  grooves  extending  back  as  far  as 
the  throat  region,  contains  the  rorquals,  fin-backs,  or  razor-backs.  B.  sib- 
Jjaldi  is  the  largest  whale,  reaching  a  length  of  85  feet.  Megaptera  includes 
the  hump-back  whales.  The  family  occurs  in  all  rocks  since  the  miocene. 
The  BAL^NID^?  or  right  whales  date  only  from  the  pliocene ;  they  have  a 
large  head,  the  ventral  surface  smooth,  the  hand  pentadactyl,  the  cervical 
vertebrae  fused,  and  no  dorsal  fin.  Balocna,  Neobalcena,  and  Rhachianectes 
belong  here. 

ORDER   VIII.     CARNIVORA    (FER^E). 

Terrestrial  or  aquatic  flesh-eating  mammals  with  unguicu- 
late  four-  or  five-toed  feet ;  incisors  usually  f,  canines  |,  strong, 
pointed  and  recurved  ;  molars  more  or  less  sectorial ;  mammae 
abdominal ;  placenta  deciduate,  almost  always  zonary. 

The  carnivora  receive  their  name  from  their  flesh-eating 
habits,  but  it  must  be  understood  that  not  every  species  con- 
forms to  this  rule,  since  some  live  largely  upon  a  vegetable  diet. 
All  have  diphyodont  and  heterodont  dentition  ;  the  first  incisor 
is  smallest,  the  third  largest.  The  canines  are  especially  charac- 
teristic ;  the  premolars  are  compressed  and  are  usually  sectorial, 
while  the  molars  are  occasionally  broad,  but  still  have  cuspidate 
crowns.  The  milk  dentition  is  usually  functional  for  a  year  after 
t>irth. 

The  feet  have  either  four  or  five  toes,  and  may  be  either 
plantigrade,  semiplantigrade,  digitigrade,  or  in  the  seals  modi- 
fied into  flippers.  The  claws  are  usually  compressed,  but  occa- 
sionally may  be  rudimentary  or  absent.  In  the  living  species 
the  brain  is  large  and  richly  convoluted,  but  in  the  creodonts  it 
\vas  much  smaller  and  nearly  smooth.  The  stomach  is  a  simple 
pear-shaped  sac  ;  the  caecum  is  small  or  absent ;  the  uterus  two- 
horned.  The  radius  and  ulna  are  always  distinct,  the  fibula 
.always  slender. 

Through  the  extinct  group  of  creodonts  the  carnivores  are 
closely  related  to  the  insectivores,  and  possibly  to  the  marsupials. 
Jn  fact  Cope  has  taken  the  creodonts  and  united  them  with  in- 


MAMMALS.  41  I 

sectivores,  and  certain  forms  here  placed  with  the  ungulates,  into 
an  order,  Bunotheria,  from  which  he  derives  the  carnivores  and 
rodents.  The  carnivores  appear  in  the  lowest  eocene  ;  the  creo- 
donts  disappear  in  the  miocene,  while  the  pinnipedia  are  first 
found  in  the  miocene. 

SUB-ORDER  i.     CREODONTA. 

Extinct  digitigrade  or  semiplantigrade  carnivores  with  small  and  scarcely 
convoluted  cerebrum  ;  incisors  f  or  f  ;  molars  never  more  than  8  ;  tail  long ; 
feet  usually  five-toed  ;  scaphoid  and  lunare  distinct. 

The  creodonts  present  marked  resemblances  to  both  marsupials  and  in- 
sectivores  in  many  structural  features,  but  they  differ  from  both  in  the  strong 
development  of  the  canines,  while  the  presence  of  a  full  milk  dentition  and 
absence  of  an  inflected  angle  of  the  lower  jaw  serve  strongly  to  mark  them  off 
from  the  former  order.  Through  the  miacidas  they  seem  connected  with  the 
canidae  {infra},  and  thus  have  given  rise  to  the  various  lines  of  living  fissi- 
pedes.  From  all  living  carnivores  they  are  marked  off  by  the  absence  of  a 
carnassial  tooth,  and  by  a  notch  or  groove  at  the  tip  of  the  distal  phalanges. 
The  OxYCLv£NiD/E  from  the  lowest  eocene  of  New  Mexico  are  known  prin- 
cipally by  the  molar  teeth.  The  ARCTOCYONID/E  occur  in  the  lower  eocene 
of  both  continents,  and  have  quadritubercular  upper  molars.  Arctocyon, 
France ;  Clanodon,  New  Mexico.  The  MESONYCHID^E  of  the  American 
eocene  have  tritubercular  molars  ;  Mesonyx.  Allied  is  the  family  LEPTIC- 
TiDjE  of  Europe  and  America  ;  Proviverra.  The  PALJEONICTID^E  {Palcecnictis, 
Patriofelis}  occurs  in  the  lower  eocene  of  Europe  and  America.  The  HY^E- 
NODONTiDvE  were  larger  animals,  much  nearer  the  recent  carnivores  (fissi- 
pedia),  but  were  distinguished  by  the  absence  of  differentiated  carnassial 
teeth,  which  however  occur  in  the  MIACID^E.  The  hyaenodontidae  range 
through  the  eocene  to  the  lower  miocene  of  both  continents  ;  the  miacidae 
have  only  been  found  in  the  eocene  of  America. 

SUB-ORDER  2.     FISSIPEDIA  (CARNIVORA  VERA). 

Digitigrade  or  plantigrade  carnivores,  with  incisors  f  (rarely  f),  premolar 
4  in  the  upper  and  molar  i  in  the  lower  jaw  sectorial,  the  other  molars  tuber- 
culate  ;  feet  four-  or  five-toed  ;  scaphoid  and  lunare  fused. 

The  carnivores  proper  are  usually  terrestrial  in  habits,  only  a  few  being 
partially  aquatic.  Most  of  them  are  carnivorous  in  diet,  but  some  are  om- 
nivorous. Correlated  with  this  flesh-eating  habit  is  the  large  size  of  the 
canines  and  the  shear-like  carnassial  or  sectorial  teeth  alluded  to  in  the  diag- 
nosis. The  brain  is  convoluted,  and  the  terminal  phalanges  are  never  notched 
at  the  tip,  but  they  are  occasionally  retractile  along  with  the  claws  they  bear. 
The  dogs  (canidae)  seem  to  be  the  central  stock  from  which  descent  has 
been  in  one  line  through  the  viverriclae  to  the  cats  and  hyaenas,  in  another 
through  the  ursidae  to  the  mustelidas.  It  must,  however,  be  mentioned  that  the 
viverridae  and  mustelidae  show  intergrading  forms.  The  sub-order  appears 


412  CLASSIFICATION  OF   VERTEBRATES. 

in  the  upper  eocene,  apparently  as  descendants  of  the  miacidae  or  palaeonic- 
tidae,  and  all  known  families  persist  at  the  present  time. 

The  CANID.E  have  typically  /  f ,  c  \,  p  \ ,  in  f  or  f ,  the  upper  sectorial 
with  two  lobes,  the  lower  with  an  inner  weaker  lobe  in  addition ;  auditory 
bulla  large,  inflated,  and  undivided  ;  feet  digitigrade,  the  fore  feet  four-  or  five- 
toed,  the  hind  feet  usually  four-toed,  claws  not  retractile.  The  majority  of 
the  species  belong  to  the  genus  Cams  (including  Lupus,  Viilpes,  etc.)  of  cos- 
mopolitan range,  including  the  dogs,  wolves,  foxes,  jackals,  etc.  Other  living 
genera  are  Octocyon  and  Lycaon  of  South  Africa.  The  fossil  genera  are  numer- 
ous in  both  continents,among  them  Temnocyon,  Amphicyon,  and  Oligobunus. 

The  URSID^E  have  plantigrade  feet,  short  and  stout  body,  sectorials 
scarcely  differentiated,  some  of  the  premolars  lost  at  an  early  date,  and  the 
auditory  bulla  flat.  Ursus,  containing  the  bears,  with  molars  f ,  is  largely 
confined  to  the  northern  hemisphere.  Mclursus  is  Asiatic.  The  fossil  gen- 
era Dinocyon,  Hyceiiarctus,  and  Arctoiherinin  form  a  line  uniting  the  dogs 
with  the  bears. 

The  PROCYONID.E  with  plantigrade  feet,  molars  f ,  tuberculate,  and  with 
the  tail  usually  ringed,  are  largely  American  ;  but  two  species,  the  raccoon 
(Procyon  lotor}  and  the  raccoon-fox  (Bassaris  astutd)  enter  the  U.  S.  The 
coati  (Nasiui)  with  long,  flexible  snout,  and  Cercoleptes  occur  in  Central 
and  South  America.  The  species  of  MUSTELID/E  are  more  numerous  ;  they 
have  the  molars  \  (|  in  Mellivora).  The  otters  (Lntra)  and  the  sea-otter 
(Enhydris}  have  webbed  feet.  Mephitis,  including  the  skunks,  is  American. 
The  badgers  (Meles}  belong  to  the  old  world,  while  the  same  common  name 
is  given  to  the  species  of  the  American  genus  Taxidea.  The  minks,  martens, 
sables,  ermines,  weasels,  and  ferrets,  belong  to  Mustela,  many  of  the  species, 
being  valuable  for  their  furs.  The  genus  begins  in  the  miocene  of  Europe. 
Gnlo,  the  wolverine,  occurs  in  the  northern  parts  of  both  hemispheres.  A 
peculiarity  of  many  of  the  mustelidce  is  the  great  development  of  anal  glands 
which  secrete  a  strong-smelling  fluid  used  as  a  weapon  of  defence. 

The  VIVERRID;E,  like  the  remaining  fissipecles,  have  a  swollen  auditory 
bulla  and  digitigrade  or  sub-plantigrade  feet.  They  have  p  f  or  |, ;//  |  or  f , 
and  usually  five  digits  on  all  the  feet.  The  species  are  all  old-world.  Cryp- 
toprocta,  Viverra,  the  civets;  Herpestes,  the  mongoose.  The  family  ap- 
pears in  the  lower  miocene.  The  HY^ENID^E,  also  an  old-world  group,  is 
closely  related  to  the  viverridae  by  its  fossil  relatives.  Hycena,  Proteles. 

The  FELID.E  have  retractile  claws,  stronly  developed  canines,  molars 
\  in  recent  species  (never  exceeding  \  in  fossils)  ;  the  upper  sectorial  with  a 
three-lobed  blade.  Felts  includes  the  majority  of  the  living  species,  —  lions, 
tigers,  leopards,  panthers,  lynxes,  pumas,  jaguars,  and  the  smaller  cats. 
CyiKelurns,  the  only  other  existing  genus,  contains  the  cheetah,  or  hunting 
leopard,  which  ranges  from  India  to  Southern  Africa.  The  family  appears 
in  the  upper  eocene  of  America,  while  species  are  found  in  the  miocene  of 
both  continents.  Among  the  extinct  genera  are  Dinictis,  Hoplophoneits,  and 
MacJuerodiis,  the  latter  characterized  by  the  enormous  canines,  these  being,, 
in  one  species,  seven  inches  in  length. 


MAMMALS. 


413 


SUB-ORDER  3.     PINNIPEDIA. 

Aquatic  pentadactyl  carnivores,  with  webbed  feet  fitted  for  swimming, 
incisors  always  less  than  f ,  p  typically  f ,  ;//  },  no  differentiated  carnassial ; 
tail  very  short. 

The  seals  and  their  allies  are  mostly  marine,  although  some  ascend 
rivers,  while  one  species  occurs  in  Lake  Baikal.  The  body  is  fitted  for  an 
aquatic  life ;  the  basal  portion  of  the  fore  limbs  is  imbedded  in  the  general 
integument,  while  the  web  of  the  toes  usually  extends  beyond  the  extremity 
of  the  clawed  digits.  The  seals  are  true  carnivores,  feeding  upon  fish,  of 
which  they  destroy  large  numbers.  The  origin  of  the  group  is  uncertain. 
The  eared  seals  show  considerable  resemblances  to  the  ursidae,  while  the  true 


FIG.  377.     Harbor  seals,  Phoca  vitulina,  after  Elliott. 

seals  suggest  an  origin  from  some  form  like  Lutra.  So  it  may  be  that  the 
group  is  polyphyletic,  or  again,  the  pinnipeds  may  have  descended  directly 
from  the  creodonts.  The  group  first  appears  with  forms  allied  to  Phoca  in 
the  miocene,  while  walrus-like  forms  occur  in  the  pliocene. 

The  OTARIID^E  or  eared  seals  have  a  small  external  ear,  and  the  soles  of 
the  feet  naked  ;  teeth  /  f ,  c  \,  p  f ,  m  \  or  \.  Otaria  includes  sea-lions 
of  the  Pacific  and  South  Atlantic ;  and  the  fur  seals,  most  familiarly  known 
from  the  northern  Pacific.  The  TRICHECHID/E,  or  walruses,  have  the  ears 
without  external  pinnae,  and  the  upper  incisors  developed  into  immense 
tusks.  The  species  of  Trichechus,  one  or  two  in  number,  are  confined  to  the 
northern  parts  of  both  oceans.  The  PHOCID.E  lack  external  ears,  have  the 
soles  of  the  feet  hairy,  the  testes  abdominal,  the  teeth  p  £,  m  ^,  and 


414.  CLASSIFICATION  OF   VERTEBRATES. 

stiff  hair  without  a  woolly  fur  beneath.  Phoca,  with  incisors  ^,  embraces  the 
common  seals  of  the  northern  Atlantic ;  MonacJius  (i  f )  ,  the  monk  seals  of 
warmer  latitudes.  Cystophora  (i  f-)  includes  the  hooded  seals  of  polar  seas 
with  inflatable  sac  connected  with  the  nostrils. 

ORDER    IX.     PRIMATES. 

Diphyodont,  heterodont  mammals,  with  typically  i  f,  m  §, 
the  molars  usually  quadritubercular ;  the  orbits  separated  from 
the  temporal  fossa  by  a  postorbital  bar  ;  clavicles  well  developed  ; 
ulna  and  radius  always  distinct  ;  feet  plantigrade,  usually  penta- 
dactyl ;  the  pollex  and  (except  in  man)  hallux  opposable  to 
the  other  digits.  The  placenta  deciduate  or  not ;  diffuse  or 
discoidal. 

The  primates,  as  a  group,  are  not  easily  denned,  especially 
if  the  extinct  forms  be  taken  into  consideration,  for  these  to  a 
great  extent  bridge  over  the  gap  which  exists,  among  recent 
forms,  between  the  primates  and  the  insectivores  and  creodonts, 
while  in  certain  points  there  are  suggestions  of  marsupial  char- 
acters. According  to  one  view,  the  order  is  polyphyletic,  the 
lemurs  having  had  one  line  of  descent,  and  the  monkeys,  apes, 
and  man  having  had  another  ancestry.  This  view  is  based 
primarily  upon  placental  structures,  but  it  is  largely  negatived 
by  the  fossil  history  so  far  as  this  is  known. 

SUB-ORDER  i.     PROSIMI/E  (LEMUROIDEA). 

Arboreal  primates  with  opposable  great  toe  ;  orbits  not  completely  sepa- 
rated from  temporal  fossa ;  mammae  thoracic,  or  thoracic  and  abdominal : 
uterus  bicornuate  ;  placenta  non-deciduate,  the  whole  surface  of  the  chorion, 
except  one  end,  being  covered  with  villi. 

The  lemurs  and  their  allies  have  their  centre  to-day  in  Madagascar,  from 
which  outlying  species  extend  to  the  African  continent  and  to  the  Indian 
archipelago,  a  distribution  which  has  suggested  a  former  continent,  '  Le- 
muria,'  in  the  Indian  Ocean.  In  former  times  their  range  was  more  extensive, 
since  abundant  remains  have  been  found  in  the  older  tertiaries  of  Europe 
and  North  America.  The  living  species  are  mostly  nocturnal,  and  many  of 
them  have  the  eyes  peculiarly  modified  in  accordance  with  their  habits.  In 
addition  to  the  characters  quoted  in  the  diagnosis  it  may  be  mentioned  that 
in  some  all  the  digits  are  clawed,  while  in  others  only  the  second  and  third 
of  the  hind  toes  are  provided  with  claws,  the  others  bearing  nails.  The 
upper  molars  have  four  or  three  tubercules,  those  of  the  lower  jaw  having  four 
or  five.  The  brain  is  but  slightly  convoluted,  and  but  slightly  overlaps  the 
cerebellum. 


MAMMALS.  415 

PACHYLEMURID.E,  z  |  to  f ,  c  \,  p  -*-,  m  |.  From  the  eocene  and  lowest 
miocene  of  Europe  and  America.  Adapts  of  Europe  is  the  best  known.  Pely- 
codus,  Tomitherium.  This  family  is  regarded  by  Cope  as  the  ancestor  of  the 
true  apes.  The  modern  lemurs  may  have  sprung  from  the  ANAPTOMOR- 
PHID.E,  in  which  the  lachrymal  foramen  lies  outside  the  orbit,  while  the  den- 
tition is  / 'I  to  §,  c  y,  p  \  to  f ,  m  \.  Anaptomorphus,  from  the  lower  eocene, 
of  Wyoming,  resembles  Tarsius  (infra}.  Necrolemur. 

The  TARSIIDVE  of  the  Indian  archipelago,  with  only  a  single  species,. 
Tarsius  spectrum,  has  a  dentition  /  f ,  c  |,  p  |,  ///  f ;  digits  2  and  3  of  the 
hind  feet  with  claws.  In  its  placentation  Tarsius  differs  from  all  other 
prosimiae  and  approaches  man.  The  lemurs  proper  belong  to  the  LEMURID^I. 
with  / f ,  c  i,  /  |  to  f ,  m  f ,  the  lower  incisors  and  canines  directed  forward,, 
and  the  first  premolar  serving  as  a  canine.  Indris,  Lemur,  Galago,  Loris^. 
etc.  The  CHIROMYID^  with  /  \,  c  §,  p  ^,  in  \ ,  contains  but  a  single  species, 
the  ave-aye,  Chiromys  madagascarensis,  which  recalls  the  rodents  in  its- 
incisors  and  diastema,  and  is  unique  in  the  greatly  elongate  middle  digit  of 
the  hand. 

Two  aberrant  families,  exhibiting  some  relationships  to  the  lemurs,  may  be 
mentioned  here.  The  first  is  NESOPITHECID^E,  based  on  the  fossil  Malagassy 
genius  Nesopithecus,  which  has  the  premolar  dentition  of  a  true  lemur,  with 
the  orbit  of  a  simian.  The  second  is  the  GALEOPITHECID/E,  with  a  single 
genus,  Galeopithecus,  from  the  East  Indies.  It  is  frequently  referred  to  the 
insectivores  with  which  it  agrees  in  its  deciduate  discoidal  placenta.  It  has 
the  fore  and  hind  limbs  connected  by  membrane  forming  a  parachute  like, 
that  of  the  flying-squirrel. 

SUB-ORDER  2.     SIMILE    (ANTHROPOIDEA). 

Arboreal  or  terrestrial  primates,  with  the  digits  (except  hapalidae)  all  witfr 
nails,  molars  with  4  or  3  tubercles  ;  orbits  completely  separated  from  temporal 
fossa  by  a  plate  of  bone  beneath  the  postorbital  bar ;  cerebrum  greatly  con- 
volute, covering  or  nearly  covering  the  cerebellum  ;  mammae  two,  always 
thoracic ;  uterus  simple  ;  placenta  discoidal,  deciduate. 

The  simiae  include  the  monkeys,  apes,  and  man,  and  the  deeper  struc- 
tural features  are  rc-inforced  by  characters  of  less-  importance.  Thus  the  eyes 
are  directed  forwards,  the  ears  are  much  as  in  man,  and  in  the  young  the 
whole  appearance  of  the  face  of  the  lower  forms  is  more  like  that  of  the 
human  being  than  is  that  of  the  adult,  the  change  being  largely  effected  by 
a  later  forward  growth  of  the  jaws.  Man  excepted,  the  sub-order  is  confined 
to  the  warmer  parts  of  both  hemispheres,  but  fossils  are  found  in  Europe  as 
far  north  as  England.  The  sub-order  has  developed  in  two  lines  in  the  two- 
hemispheres,  the  platyrhine  forms  belonging  to  the  new  world,  the  catarrhine 
to  the  old. 

In  the  PLATYRHINI  the  nostrils  are  separated  by  a  wide  septum,  and  are 
directed  outwards.  The  HAPALID/E  have  /'  f ,  c  \,p  f , ;;/  f  ;  all  digits,  except 
the  great  toe,  furnished  with  claws,  and  the  tail  non-prehensile.  The  species 
belonging  to  the  genera  Hapale  and  Midas  have  the  common  name  of 
marmosets.  Apparently  this  family  has  descended  from  the  CEBID^,  ia. 


41 6  CLASSIFICATION  OF   VERTEBRATES. 

which  the  tail  is  frequently  prehensile,  and  the  premolars  are  |.  Mycetes  in- 
cludes the  howling  monkeys  ;  Pithecia,  the  sakis  ;  Ateles,  the  spider  monkeys  ; 
Cebus,  the  sapajous,  species  of  which  frequently  accompany  the  organ- 
grinder.  Homujiculus  and  Anthropops  occur  in  the  tertiary  of  Patagonia. 

The  CATARRHINI  have  the  nasal  septum  narrow,  the  nostrils  directed 
forwards,  and  a  dentition  /  f ,  c  \,  p  f ,  m  f .  In  the  CERCOPITHECID^:  (Cy- 
nopithecidae)  the  tail  is  usually  long,  the  molars  quadrituberculate,  and  the 
ischial  region  presents  callosities.  All  of  the  tailed  apes  of  the  old  world 
belong  to  this  family.  Cynocephalus  contains  the  baboons  of  Africa,  in  which 
the  tail  is  of  moderate  size,  and  the  maxillary  bones  swollen.  Here  also 
belong  the  drill  and  mandrill.  The  macaques  (Macacus),  are  almost  entirely 


FIG.  378.     Chimpanzee,  Troglodytes  niger,  after  Brehm. 

Asiatic,  one  species  entering  Europe  at  Gibraltar.  Cercopitliecus.  Semno- 
pithecus.  The  SIMIID,E,  or  ANTHROPOMORPHA,  contains  apes  in  which  the 
tail  is  lacking,  the  anterior  limbs  longer  than  the  posterior,  and  ischial  cal- 
losities lacking,  except  in  Hylobates,  which  includes  the  gibbons  of  Asia,  in 
which  the  arms  are  so  long  that  they  reach  the  ground  when  the  animal  is  in 
an  upright  position.  Simia  includes  the  orang-utan  of  Sumatra  and  Borneo, 
in  which  the  great  toe  is  small,  the  arms  long,  and  the  ribs,  12  pairs.  Gorilla 
of  Africa  has  13  pairs  of  ribs,  and  prominent  superciliary  ridges.  Troglodytes 
includes  the  chimpanzees,  of  which  there  are  one  or  two  species,  in  which 
the  ribs  are  as  in  gorilla.  They  come  from  western  Africa. 

The  HOMINIDJE,  or  BIMANA,  includes  man,  who  is  far  less  remote  from 
the  simiidae  than  these  are  from  the  new-world  monkeys.  The  chief  charac- 
ters are  the  upright  position,  the  lack  of  opposable  toe,  the  'enormous  size 
of  the  brain,  correlated  with  his  mental  development,  and  the  distribution  of 


MAMMALS.  417 

hair  upon  the  body,  it  being  best  developed  in  those  places  where  it  is  most 
sparse  in  the  allied  forms. 

Man  presents  certain  features  in  which  he  resembles  more  closely  each 
of  the  anthropomorphous  apes,  while  in  others  he  differs  from  them  all,  so 
that  it  is  difficult  to  say  which  is  his  nearer  relative.  Of  the  genus  Homo 
there  is,  according  to  accepted  tests,  but  a  single  species ;  but  the  question 
of  arrangement  of  the  races  affords  far  more  difficulties.  For  all  such  discus- 
sions reference  must  be  made  to  the  works  on  anthropology.  The  age  of 
man  on  the  earth  is  another  question  which  can  only  be  alluded  to  here. 
That  man  has  been  here  far  longer  than  the  seven  or  eight  thousand  years 
of  history  is  now  beyond  a  doubt.  His  remains  and  his  handiwork  date  back 
to  a  time  far  before  any  records  or  any  traditions  ;  to  a  time  when  the  mam- 
moth was  alive,  and  when  England  and  continental  Europe  had  a  fauna 
recalling  those  of  the  tropics  to-day  ;  when  the  mastodon,  Glyptodon,  and 
Megatherium  ranged  in  South  America.  But  when  we  attempt  to  pass  back 
of  the  pleistocene  the  evidence  is  scanty,  and  not  beyond  question.  The 
skull  of  Calaveras  and  the  '  Pithecanthropus  erectus '  of  Java,  like  the  miocene 
flint  chips  of  Thenay,  need  more  evidence  in  their  support  before  they  can 
be  accepted  as  proving  the  existence  of  man  in  tertiary  time,  no  matter  how 
probable  such  existence  may  be  upon  a  priori  grounds. 


NDEX 


Aard  vark,  382. 
Abdominal  pores,  107. 
Abdominal  vein,  197. 
Abducens  nerve,  61. 
Abomasum,  35. 
Acanthias,  239. 
Acanthodes,  251. 
Acanthoderma,  266. 
Acanthodidse,  251. 
Acanthoglossus,  377. 
Acanthopteri,  258. 
Acanthopterygii,  258. 
Accessory  of  Willis,  64. 
Accipiter,  348. 
Accipitres,  348. 
Aceratherium,  395. 
Acetabular  bone,  171. 
Acetabulum,  170. 
Achaenodon,  397. 
Achirus,  265. 
Acinose  glands,  90. 
Acipenser,  251. 
Acipenseridae,  250. 
Acris,  287. 
Acrodont  teeth,  294. 
Actinistia,  249. 
Actinosts,  175. 
Actinotrichia,  174. 
Adapis,  415. 
Adapisoricidae,  385. 
Adder,  325. 
Adipose  tissue,  14. 
Adrenal,  131. 
yEgitognathas,  351. 
^Egithognathous,  335. 
^piornis,  346. 
^piornithes,  346. 
.'Etosaurus,  328. 
Afferent  nerves,  47. 
Agamidae,  320. 
Agathaumas,  317. 
Agkistrodon,  326. 
Aglossa,  286. 
Aglyphodonta,  325. 


Agnatha,  219. 
Agonidae,  260. 
Agriochoerus,  398. 
Aguti,  391. 
Air  bladder,  25. 
Air  sacs,  31. 
Aistopoda,  283. 
Ala  spuria,  330. 
Albatross,  349. 
Albulia,  256. 
Albumen,  207. 
Alca,  349. 
Alcedo,  348. 
Alces,  399. 
Alecithal  eggs,  206. 
Alectoromorphae,  350. 
Alepocephalidae,  256. 
Alewife,  256. 
Alisphenoid,  158. 
Allantoic  artery,  190. 
Allantoic  placenta,  374. 
Allantoidea,  288. 
Allantois,  373. 
Alligator,  328. 
Alligator  gar,  252. 
Alligatoridae,  328. 
Alligator  snapper,  311. 
Allotheria,  377. 
Alopiidas,  239. 
Alpaca,  398. 
Altrices,  330. 
Alula,  330. 
Alutera,  266. 
Alveoli,  20. 
Alveoli  of  lung,  30. 
Alytes,  287. 
Amber  fish,  263. 
Amblypoda,  400. 
Amblyopsidae,  257. 
Amblyopsis,  257. 
Amblyrhiza,  391. 
Amblystoma,  285. 
Amia,  252. 
Amiidae,  252. 

419 


Amiurus,  255. 
Ammocoetes,  222. 
Ammodytes,  263. 
Ammodytidae,  262. 
Ammodytoidea,  262. 
Amnion,  288. 
Amniota,  288. 
Amphibia,  274. 
Amphiccelias,  315. 
Amphicoelous,  138. 
Amphicyon,  412. 
Amphilestes,  380. 
Amphioxus,  2. 
Amphiplaga,  259. 
Amphiplatyan,  140. 
Amphisbaena,  321. 
Amphistylic  skull,  234. 
Amphitherium,  380. 
Amphiuma,  285. 
Amphiumidas,  285. 
Ampullae,  68,  70. 
Amyda,  310. 
Amyzon,  255. 
Anabas,  262. 
Anableps,  257. 
Anacanthini,  264. 
Anaconda,  325. 
Anal  fin,  177,  228. 
Anallantoidea,  226. 
Anamnia,  226. 
Anapophysis,  141. 
Anaptomorphidas,  415. 
Anaptomorphus,  415. 
Anarrhichas,  261. 
Anarrhichidae,  261. 
Anas,  348. 
Andrias,  285. 
Angler,  266. 
Anguilla,  257. 
Anguillidae,  256. 
Anguis,  320. 
Angulare,  164. 
Anhima,  348. 
Anhinga,  347. 


420 


INDEX. 


Animalivora,  387. 

Archaeopteryx,  343. 

Autostylic  skull,  241. 

Annulata,  321. 

Archaeornithes,  343. 

Auxis,  263. 

Anolis,  320. 

Archegosaurus,  283. 

Aves,  330. 

Anomaluridae,  330. 

Archenteron,  6. 

Axillary  artery,  189. 

Anomalurus,  390. 

Archer  fish,  262. 

Axis,  142. 

Anomodontia,  305. 

Archipterygium,  172. 

Axis  cylinder,  n. 

Anoplotheridae,  398. 

Arcifera,  286. 

Axis  deer,  399. 

Anoplotherium,  398. 

Arctocyon,  411. 

Axon,  ii. 

Anser,  348. 

Arctocyonidae,  411. 

Aye-Aye,  415. 

Ant-eater,  scaly,  382. 

Arctomys,  390. 

Azygos  vein,  196. 

Ant-eater,  spiny,  377. 

Arctotherium,  412. 

Ant-eaters,  true,  383. 

Ardea,  348. 

Babirusa,  397. 

Antebrachium,  176. 

Area  opaca,  342. 

Baboon,  416. 

Antennariidae,  266. 

Area  pellucida,  342. 

Bactrian,  398. 

Antennarius,  266. 

Areolar  tissue,  13. 

Badger,  412. 

Anterior     abdominal     vein, 

Armadillo,  383. 

Balaena,  410. 

197. 

Arterial  blood,  184. 

Balsenidae,  410. 

Anthracotheriidae,  397. 

Arteries,  178,  188. 

Balaenoidea,  409. 

Anthracotherium,  397. 

Arthrodira,  271. 

Balaenoptera,  410. 

Anthropoidea,  415. 

Articular  process,  140. 

Balaenopteridae,  410. 

Anthropomorpha,  416. 

Articulary,  158. 

Balancers,  24,  282. 

Anthropops,  416. 

Artiodactyla,  396. 

Balanoglossus,3. 

Antiarcha,  225. 

Arvicola,  390. 

Baleen,  409. 

Antidorcas,  399. 

Arytenoid  cartilage,  28. 

Balistes,  266. 

Antilocapra,  399. 

Ascalabotae,  319. 

Bandicoot,  380. 

Antilope,  399. 

Ascalabotes,  319. 

Banteng,  400. 

Antrozous,  387. 

Asineops,  262. 

Baphetes,  284. 

Ant-shrike,  351. 

Asp,  325. 

Baptanodon,  313. 

Anura,  286. 

Aspidocephali,  225. 

Barbus,  255. 

Aorta,  181. 

Aspidonectes,  310. 

Barracuda,  262. 

Aortic  arches,  182,  185. 

Aspidorhynchus,  252. 

Basalia,  173. 

Apatornis,  344. 

Ass,  395. 

Bascanion,  325. 

Apeltes,  258. 

Asterolepis,  225. 

Basibranchial,  154. 

Apes,  416. 

Asterospondyli,  238. 

Basihyal,  155. 

Aphredoderus,  259. 

Asterospondylous  vertebras, 

Basilosaurus,  408. 

Apodes,  256. 

234- 

Basioccipital,  157. 

Aponeurosis,  112. 

Astragalus,  177. 

Basisphenoid,  158. 

Appendage,  pyloric,  38. 

Atalapha,  387. 

Basitemporal,  334. 

Appendicularia,  2. 

Atelodus,  395. 

Bassaris,  412. 

Appendicular  skeleton,  167. 

Athecse,  310. 

Bass,  black,  259. 

Appendix,  digitiform,  36. 

Atherina,  262. 

Bat  fish,  266. 

Appendix  vermiformis,  39. 

Atherinidae,  262. 

Bathyergus,  390. 

Aptenodytes,  347. 

Atlantosaurus,  315. 

Bathyergidae,  390. 

Apteryges,  346. 

Atlas,  142. 

Batoidea,  239. 

Apteryx,  346. 

Atoposauridas,  328. 

Batrachia,  274. 

Apteria,  95. 

Atrium,  181. 

Batrachidae,  266. 

Aqueduct  of  brain,  50. 

Auchenaspis,  225. 

Batrachus,  266. 

Aqueductus  vestibuli,  70. 

Auchenia,  398. 

Bats,  386. 

Aqueous  humor,  83. 

Auditory  bulla,  357. 

Bdellostoma,  224. 

Aquila,  348. 

Auditory  nerve,  63. 

Bdellostomidns,  224. 

Arachnoid  membrane,  57. 

Auditory  ossicles,  158. 

Bead  snake,  325. 

Arapaima,  256. 

Auk,  349. 

Bears,  412. 

Arcades,  298. 

Auricle,  181,  184. 

Beaver,  390. 

Archaeoceti,  408. 

Aurochs,  400. 

Bee-eater,  348. 

INDEX. 


421 


Belideus,  380.                            1   Brachium,  176. 

Callionymus,  263. 

Belodon,328. 

Bradipodidae,  383. 

Callopterus,  252. 

Belone,  257. 

Bradypus,  383. 

Callorhynchus,  242. 

Belonida;,  257. 

Brain,  48. 

Camarasaurus,  315. 

Belonorhvnchus,  258. 

Bramidae,  263. 

Camelidae,  398. 

Berycidae,  261. 

Branchiae,  22. 

Camelopardalis,  399. 

Berycoidea,  261. 

Branchial  arches,  154. 

Camels,  398. 

Beryx,  261. 

Branchial  clefts,  22. 

Camelus,  398. 

Bettongia,  380. 

Branchial  rays,  156. 

Campanula  Halleri,  231. 

Bibos,  400. 

Branch  iosaurus,  283. 

Camper's  angle,  358. 

Bicuspids,  366. 

Branchiostegal  rays,  156. 

Canaliculi,  15. 

Bill  fish,  257. 

Breast  bone,  147. 

Canidae,  412. 

Bimana,  416. 

Breathing  valves,  253. 

Canines,  364. 

Bipolar  nerve  cells,  10. 

Brevilinguia,  320. 

Canis,  412. 

Birds,  330. 

Brevirostres,  328. 

Cannon  bone,  394. 

Birds  of  Paradise,  350. 

Brevoortia,  256. 

Capra,  399. 

Bison,  400. 

Broadbill,  351. 

Caprimulga,  348. 

Bittern,  348. 

Bronchi,  27. 

Capybara,  391. 

Black  bass,  259. 

Bronchiole,  30. 

Carangidae,  263. 

Black  fish,  408. 

Brontosaurus,  315. 

Caranx,  263. 

Black  snake,  325. 

Brontotherium,  396. 

Carapace,  93,  308. 

Blarina,  385. 

Bruta,  381. 

Carassius,  255. 

Blastoderm,  211. 

Bubalus,  400. 

Carcharias,  239. 

Blastodermic  vesicle,  214. 

Bubo,  348. 

Carcharinus,  239. 

Blastomeryx,  399. 

Buccalis  nerve,  62. 

Carcharodon,  239. 

Blastopore,  6. 

Bucco,  348. 

Cardiac  glands,  367. 

Blennidae,  261. 

Buceros,  348. 

Cardiac  region,  34. 

Blennioidea,  261. 

Buffalo,  400. 

Cardinal  sinus,  195. 

Blennius,  261. 

Bufo,  286. 

Cardinal  vein,  183. 

Blind  fish,  257. 

Bufonidce,  286. 

Carettochelydae,  311. 

Blood,  16. 

Bulbus  arteriosus,  181. 

Cariacus,  399. 

Bluefish,  263. 

Bull  frog,  287. 

Caribou,  399. 

Boa,  325. 

Bull  head,  255. 

Carina,  148. 

Boar,  397. 

Bunodont,  365. 

Carinatae,  149. 

Body  of  vertebra,  135. 

Bunotheria,  403,  411. 

Carnassial  teeth,  411. 

Bombinator,  287. 

Burbot,  264. 

Carnivora,  410. 

Bonasa,  350. 

Burdach's  column,  54. 

Carotid,  183. 

Bone,  14. 

Bursa  Fabricii,  39. 

Carp,  255. 

Bone,  membrane,  15. 

Bustard,  349. 

Carpale,  176. 

Bone,  cartilage,  15. 

Butter  fish,  263. 

Carpus,  176. 

Bony  fishes,  252. 

Butterfly  fish,  262. 

Cartilage,  14. 

Bony  gar,  257. 

Buteo,  348. 

Cartilage  bone,  15. 

Booted  tarsus,  331. 

Buzzard,  348. 

Casserian  ganglion,  61. 

Borers,  224. 

Cassowary,  346. 

Bos,  400. 

Cacatua,  349. 

Castor,  390. 

Botaurus,  348. 

Caeciliae,  287. 

Castoridas,  390. 

Bothremys,  311. 

Caecilians,  288. 

Castoroides,  391. 

Bothriolepis,  225. 

Caecomorphas,  349. 

Casuaridas,  346. 

Bothrops,  326. 

Caecum,  intestinal,  39. 

Casuarius,  346. 

Bow  fin,  252. 

Caenolestes,  380. 

Cataphracta,  326. 

Bowman's  capsule,  119. 

Caenotherium,  398. 

Cataphracti,  259. 

Box  turtle,  311. 

Calamodon,  403. 

Catarrhini,  416. 

Brachial  artery,  189. 

Calamoichthys,  250. 

Cat  fish,  255. 

brachial  plexus,  48. 

Calcaneum,  177. 

Cathartes,  348. 

422 


INDEX. 


Catoblephas,  399. 

Catopterus,  252. 

Catostomidae,  255. 

Catostomus,  255. 

Cats,  412. 

Cattle,  400. 

Caturus,  252. 

Cauda  equina,  48. 

Caudal  artery,  183. 

Caudal  fin,  177,  228. 

Caudal  region,  142. 

Caudal  vein,  192. 

Cavia,  391. 

Caviare,  251. 

Caviidae,  391. 

Cavicornia,  399. 

Cebidae,  415. 

Cebus,4i6. 

Cement,  19. 

Centetidae,  385. 

Centrale,  176. 

Centres  of   ossification,   15, 

133- 

Centriscidae,  258. 
Centrum,  135. 
Cephalaspis,  225. 
Cephalochordia,  2. 
Cephalodiscus,  3. 
Ceratobranchial,  154. 
Ceratodus,  273. 
Ceratohyal,  155. 
Ceratophrys,  274. 
Ceratops,  317. 
Ceratopsia,  316. 
Ceratorhinus,  395. 
Ceratosaurus,  316. 
Cercoleptes,  412. 
Cercopithecidae,  416. 
Cercopithecus,  416. 
Cerebellum,  50. 
Cerebral  hemispheres,  49. 
Cerebrum,  49. 
Cervicornia,  398. 
Cervulus,  399. 
Cervus,  399. 
Cestracionidas,  238. 
Cetacea,  405. 
Cervical  plexus,  48. 
Cervical  region,  142. 
Chsenomorphae,  347. 
Chaeropus,  398. 
Chaetodon,  262. 
Chaetodontidae,  262. 
Chalaza,  207. 


Chalicotheriidas,  396. 
Chamaeleon,  319,  320. 
Chamois,  399. 
Champosaurus,  314. 
Characinidae,  255. 
Characinus,  255. 
Charadrius,  349. 
Chauliodus,  256. 
Cheetah,  412. 
Chelone,  310. 
Chelonia,  307. 
Chelonidae,  310. 
Chelopus,  311. 
Chelydosauria,  310. 
Chelydra,  311. 
Chelydridae,  311. 
Chelys,  311. 
Cheropus,  380. 
Chevrotains,  398. 
Chiasma,  optic,  61. 
Chiasmodontidae,  261. 
Chilomycterus,  267. 
Chilonycteris,  388 
Chimaera,  242. 
Chimpanzee,  416. 
Chinchilla,  391. 
Chinchillidae,  391. 
Chipmunk,  390. 
Chiromyidae,  415.- 
Chiromys,  415. 
Chironectes,  579. 
Chiroptera,  386. 
Chirox,  378. 
Chlamydophorus,  383. 
Chlamydosaurus,  320. 
Chlamydoselachidae,  238, 
Chlamydoselachus,  238. 
Chlamydotherium,  384. 
Choana,  76. 
Chceropotamus,  397. 
Choloepus,  383. 
Chologaster,  257. 
Chondrin,  133. 
Chondrocranium,  150. 
Chondropterygii,  232. 
Chondrostei,  250. 
Chordata,  i. 
Chorda  tympani,  63. 
Chordediles,  348. 
Chorion,  288,  373. 
Chorionic  placenta,  374. 
Chorion  laeve,  375. 
Choristodera,  314. 
Choroid  fissure,  79. 


Choroid  plexus,  52,  54. 
Chorophilus,  287. 
Chromatophores,  91. 
Chromidae,  261. 
Chromosomes,  208. 
Chrysemys,  311. 
Chrysochloridas,  385. 
Ciconia,  348. 
Cilia,  10. 

Ciliated  epithelium,  10. 
Ciliary  ganglion,  62. 
Ciliary  muscles,  83. 
Ciliary  process,  83. 
Cimolichthys,  257. 
Cimoliosaurus,  306. 
Cingulum,  365. 
Circulatory  organs,  178. 
Cistudo,  311. 
Civet,  412. 
Cladistia,  250, 
Cladodus,  237. 
Cladoselache,  237. 
Cladoselachii,  237. 
Clasnodpn,  411. 
Clarias,  255. 
Claspers,  209,  235. 
Clavicle,  169,  170. 
Claws,  99. 
Cleithrum,  169. 
Clepsydrops,  306. 
Clevelandia,  263. 
Clidastes,  322. 
Climbing  perch,  262. 
Cloaca,  39. 
Clupeidae,  256. 
Clupea,  256. 
Cnemidophorus,  321. 
Coati,  412. 
Cobitis,  255. 
Cobra,  325. 
Coccosteus,  272. 
Coccygomorphae,  348. 
Cochlea,  73. 
Cod ,  264. 

Coelacanthidas,  249. 
Coelacanthus,  249. 
Coeliac  axis,  190. 
Coelogenys,  391. 
Coelom,  7,  101. 
Coffin  bone,  394. 
Colaptes,  351. 
Colius,  348. 
Colon,  38,  368. 
Colossochelys,  311. 


Colosteus,  283. 

J.  J.  V  J-SJ^SL, 

Coturnix,  350. 

^J 

Cyclodipterini,  249. 

Colubridae,  325. 

Cotyledonary  placenta,  373. 

Cyclodus,  320. 

Colubriformia,  324. 

Cotylophora,  396. 

Cycloid  scales,  228. 

Columba,  350. 

Cotylosauria,  304. 

Cyclopterus,  260. 

Columella,  74,  159. 

Coverts,  330. 

Cyclospondyli,  239. 

Columnar  epithelium,  9. 

Cowper's  glands,  371. 

Cyclospondylous    vertebrae,*' 

Columns  of  cord,  44. 

Coypu,  391. 

234- 

Coly,  348. 

Cramp  fish,  239. 

Cyclostomata,  219. 

/         Colymbus,  349. 

Crane,  349. 

jCyclotura,  383. 

Commissures  of  brain,  55. 

Cranial  nerves,  58. 

Cygnus,  348. 

Compsognathus,  316. 

Cranial  vertebrae,  166. 

Cymatogaster,  260. 

Concrescence,  213. 

Cranium,  150. 

Cynaelurus,  412. 

Condylarthra,  393. 

Crax,  350. 

Cynocephalus,  416. 

Condylura,  385. 

Crassilingua,  319. 

Cynomys,  390. 

Cone  cells,  79. 

Cremaster  muscle,  371. 

Cynopithethidae,  411. 

Cone,  in  teeth,  365. 

Creodonta,  411. 

Cynoscion,  259. 

Conid,  365. 

Crevalle,  263. 

Cyprinidae,  255. 

Coney,  402. 

Cricetus,  390. 

Cyprinodon,  257. 

Conger,  257. 

Cricoid  cartilage,  28. 

Cyprinodontidae,  257. 

Congo  eel,  285. 

Cricotus,  283. 

Cyprinus,  255. 

Conjunctiva,  81. 

Crista  acustica,  71. 

Cystic  duct,  40. 

Connective  tissues,  12. 

Crista  galli,  356. 

Cystophora,  414. 

Conodonts,  223. 

Crocidura,  385. 

Conurus,  349. 

Crocodilia,  326. 

Dactylethra,  286. 

Conus  arteriosus,  181. 

Crocodilidse,  328. 

Dactylopteridae,  260. 

Convolutions  of  brain,  52. 

Crocodilus,  328. 

Dactylopterus,  260. 

Copelatae,  2. 

Crop,  34. 

Dactyloscopus,  261. 

Copperhead,  326. 

Crossopterygii,  249. 

Darter  (bird),  347. 

Copula,  154. 

Crotalidae,  325. 

Darters  (fish),  259. 

-    Coracias,  348. 

Crotalus,  325. 

Dasyatis,  240. 

Coracoid  bone,  168,  169. 

Crura  cerebri,  53. 

Dasypaedes,  330. 

Coracoid  process,  359. 

Cryptacanthodes,  261. 

Dasypodidae,  383. 

Coral  snake,  325. 

Cryptobranchia,  285. 

Dasyprocta,  391. 

Coregonus,  256. 

Cryptobranchidae,  285. 

Dasyproctidae,  391. 

Corium,  87. 

Cryptobranchus,  285. 

Dasypus,  383. 

Cormorant,  347. 

Cryptodira,  310. 

Dasyuridae,  379. 

Cornua  of  cord,  44. 

Cryptoprocta,  412. 

Dasyurus,  379. 

Cornu  trabeculae,  152. 

Crypturi,  346. 

Decidua,  374. 

Coronary  bone,  394. 

Ctenacodon,  378. 

Decidua  reflexa,  375. 

Corpora  bigemina,  50,  53. 

Ctenodactylus,  391. 

Decidua  serotina,  375. 

Corpora    quadrigemina,   50, 

Ctenodus,  273. 

Decidua  vera,  375. 

53- 

Ctenoid  scales,  228. 

Decussation,  56. 

Corpus  albicans,  53. 

Ctenolabrus,  260. 

Deer,  399. 

Corpus  callosum,  56. 

Cubical  epithelium,  9. 

Delphinapterus,  408. 

Corpus  mammilare,  53. 

Cuboid  bone,  177. 

Delphinidoe,  408. 

Corpus  restiforme,  54. 

Cuckoo,  348. 

Delphinoidea,  408. 

Corpus  striatum,  51. 

Cuculus,  348. 

Delphinus,  408. 

Cortex  of  brain,  51. 

Cuneiform  bone,  177. 

Demibranch,  22. 

Cortis'  organ,  73. 

Gunner,  260. 

Dendrites,  n. 

Coryphaenidae,  263. 

Currasow,  350. 

Dendrobatidae,  287. 

Coryphodon,  400. 

Cuscus,  380. 

Dendrolagus,  380. 

Cosoryx,  399. 

Cuticular  layer,  87. 

Dental  formula,  366. 

Cottidae,  259. 

Cutis,  87. 

Dental  papilla,  19. 

Cottus,  259. 

Cutlas  fish,  263. 

Dental  ridge,  19. 

424 


INDEX. 


Dentary,  164. 

Diphycercal  fin,  229. 

Denticetae,  408. 

Diphyodont,  366. 

Dentinal  canals,  16. 

Diplarthra,  392. 

Dentinal  papilla,  92. 

Diplodocus,  316. 

Dentine,  16,  19. 

Diplosaurus,  328. 

Dercetidae,  258. 

Diplospondyli,  238. 

Derma,  87. 

Diplurus,  249. 

Dermal  glands,  89. 

Dipneumonia,  273. 

Dermal  organs,  86. 

Dipneustes,  267. 

Dermaptera,  385. 

Dipnoi,  267. 

Dermochelys,  310. 

Dipodidae,  390. 

Derotremata,  285. 

Diprotodon,  380. 

Desmodus,  388. 

Diprotodonta,  380. 

Desmognathae,  347. 

Diprotodontidae,  380. 

Desmognathus,  285. 

Dipsas,  325. 

Desmognathous  skull,  334. 

Dipterus,  273. 

Deutoplasm,  205. 

Dipus,  390. 

Devil  fish,  240. 

Discobolus,  260. 

Devexa,  399. 

Discocephali,  263. 

Diaphragm,  106. 

Discoidal  placenta,  373. 

Diapophysis,  141. 

Discus  proligerus,  125. 

Diatryma,  346. 

Ditrema,  260. 

Diceratherium,  395. 

Dinosauria,  314. 

Dichobune,  398. 

Doctor  fish,  262. 

Diclonius,  317. 

Dodo,  350. 

Dicotyles,  397. 

Dog,  412. 

Dicotylidae,  397. 

Dog  fish,  239. 

Dicrocynodon,  380. 

Dog  sharks,  239. 

Dicynodon,  305. 

Dolichosauria,  321. 

Didelphia,  378. 

Dolichosoma,  283. 

Didelphidae,  379. 

Dolphins  (mammals),  408. 

Didelphys,  379. 

Dolphin  (fish),  263. 

Didymodus,  237. 

Dorcatherium,  398. 

Didunculus,  350. 

Dorosoma,  256. 

Didus,  350. 

Dorsal  aorta,  182. 

Diemyctylus,  285. 

Dorsal  fin,  177,  228. 

Diencephalon  ,  49. 

Dorsal  nerve  roots,  46. 

Diffuse  placenta,  373. 

Dorsal  region,  142. 

Digestive  tract,  34. 

Dove,  350. 

Digitiform  appendix,  36. 

Draco,  320. 

Digitigrade,  361. 

Drill,  416. 

Digits,  176. 

Dromaeognathi,  345. 

Dimetrodon,  306. 

Dromaeognathous  skull,  334. 

Dimorphodon,  330. 

Dromaius,  346. 

Dimylidae,  385. 

Dromatherium,  377. 

Dinichthys,  272. 

Dromedary,  398. 

Dinictis,  412. 

Dryolestes,  380. 

Dinoceras,  400. 

Duckbill,  376. 

Dinocyon,  412. 

Ducks,  348. 

Dinornithidae,  346. 

Duct,  cystic,  40. 

Dinotheridae,40i. 

"      Gartner's,  129. 

Dinotherium,40l. 

"      hepatic,  41. 

Diodon,  267. 

"      mesonephric,  119. 

Diomedia,  349. 

"      Miillerian,  126. 

Duct,  Stenson's,  77. 

"      urogenital,  126. 
pronephric,  117. 

"     Wolffian,  119,  126. 

"      of  Wirsung,  40. 
Ductus  Botallii,  187. 
Ductus  choledochus,  41. 
Ductus  Cuvierii,  183. 
Ductus  endolymphaticus,  70. 
Dugong,  405. 
Duodenal  artery,  190. 
Duodeno-hepatic  omentum, 

105. 

Duodenum,  35. 
Duplicidentata,  391. 
Dura  mater,  57. 

Eagle,  348. 

Eared  seal,  413. 

Ears,  69. 

Ear  stones,  71. 

Echidna,  377. 

Echidnidae,  377. 

Echineis,  264. 

Ectethmoid,  244. 

Ectoderm,  6. 

Ectodermal  structures,  43.. 

Edaphodon,  242. 

Edentata,  381. 

Educabilia,  361. 

Eel,  Congo,  285. 

Eel,  mud,  285. 

Eel  pouts,  261. 

Eels,  256. 

Efferent  nerves,  47. 

Egg,  5- 

Egg,  development  of,  205. 
Egg  tooth,  343. 
Elapidae,  325. 
Elaphus,  399. 
Elaps,  325. 
Elasmobranchii,  232. 
Elassoma,  259. 
Elastic  tissue,  14. 
Elastica  exerna,  135. 
Elastica  interna,  134. 
Electrical  organs,  115. 
Electric  eel,  255. 
Electric  skate,  239. 
Elephant,  401. 
Elephantidae,  401. 
Elephas,  401. 
Elginia,  305. 
Elotherium,  397. 


INDEX. 


425 


Emballonura,  388. 
Emballonuridae,  387. 
Embiotoca,  260. 
Embiotocidae,  260. 
Embolomeri,  283. 
Embolomerous,  136. 
Emeu,  346. 
Empedias,  304. 
Emperor  fish,  262. 
Emydas,  311. 
Enamel,  19. 
Enamel  organ,  19. 
End  buds,  68. 
Endolymph,  71. 
Endolymphatic  duct,  70. 
Endothelium,  9. 
Engraulis,  256. 
Engystoma,  287. 
Engystomidae,  287. 
Enhydris,  412. 
Ensiform  process,  149,  356. 
Entelops,  383. 
Enteroccele,  7. 
Enteropneusti,  3. 
Entepicondylar        foramen, 

304- 

Entoderm,  6. 
Entodermal  organs,  17. 
Entoglossum,  335. 
Entoplastron,  308. 
Eohippus,  395. 
Epiaxial  muscles,  109. 
Epiblast,  6. 
Epibranchial,  154. 
Epicentrals,  144,  246. 
Epicrele,  50. 
Epicoracoid,  170. 
Epidermal  structures,  87. 
Epidermis,  87. 
Epididymis,  130. 
Epiglottis,  370. 
Epihyal,  155. 
Epimere,  101. 
Epimerals,  144,  246. 
Epiotic,  158. 
Epiphysis,  85,  134,  355. 
Epiplastron,  308. 
Epipleurals,  144,  246. 
Epipubis,  171. 
Episternum,  149,  308. 
Epistropheus,  142. 
Epithelium,  9. 
Epitrichium,  88. 
Epoophoron,  129. 


Equidae,  395. 
Equus,  395. 
Erethyzon,  391. 
Eretmochelys,  310. 
Erinaceidae,  385. 
Erinaceus,  385. 
Erismatopterus,  256. 
Ermine,  412. 
Eryops,  283. 
Erythrinus,  255. 
Eschatius,  398. 
Esocidae,  257. 
Esox, 257. 
Esthonyx,  403. 
Etheostoma,  259. 
Ethmoid,  357. 
Ethmoid  plate,  152. 
Eumeces,  320. 
Eumylodus,  242. 
Eunectes,  325. 
Euornithes,  347. 
Eurhipidurae,  345. 
Eurycormus,  252. 
Eurylaima,  351. 
Eurylepis,  251. 
Eurypharynx,  257. 
Euselachii,  238. 
Eustachian  tube,  73. 
Eusthenopteron,  249. 
Eusuchia,  328. 
Eutainia,  325. 
Eutheria,  378. 
Exoccipital,  157. 
Exocoetus,  257. 
Exocoetidae,  257. 
Exoskeleton,  91. 
External  carotid,  183. 
External  ear,  24. 
External  gills,  24. 
Eye,  pineal  or  parietal,  85. 
Eyes,  78. 

Facial  angle,  358. 
Falciform  process,  231. 
Falco,  348. 
Fallopian  tube,  127. 
Fascia,  112. 

Fasciculus  cuneatus,  54. 
Fasciculus  gracilis,  54. 
Fat,  13. 

Feather  tracts,  95. 
Feathers,  94,  330. 
Felidae,  412. 
Felis,  412. 


Felsinotherium,  405. 
Femoral  artery,  191. 
Femur,  176. 

Fenestra  ovalis,  72,  159. 
Fenestra  rotunda,  72. 
Fer-de-lance,  326. 
Ferret,  412. 
Fertilization,  5. 
Ferae,  410. 
Fetterbone,  394. 
Fiber,  390. 
Fibrillations,  10. 
Fibula,  176. 
Fibulare,  176. 
Fierasfer,  261. 
Fierasferidae,  261. 
Fifth  ventricle,  57. 
File  fish,  266. 
Filoplumes,  95. 
Filum  terminale,  48. 
Fin-back  whale,  410. 
Fins,  167,  177,  228,  229. 
Firmis;ernia,  287. 
Fishes,  228. 
Fissilinguia,  320. 
Fissipedia,  411. 
Fistularia,  258. 
Fistularidae,  258. 
Flamingo,  348. 
Flat  fishes,  264. 
Flexures  of  brain,  56. 
Floccular  lobes,  338. 
Flounder,  265. 
Flying  fish,  257,  260. 
Flying  fox,  388. 
Flying  squirrel,  390. 
Fodientia,  382. 
Fontanelle,  157,  234. 
Food  yolk,  205. 
Foramen  jugulare,  166. 
Foramen  lacerus,  164. 
Foramen  magnum,  157. 
Foramen  of  Monro,  50. 
Foramen,  obturator,  170. 
Foramen  ovale,  165. 
Foramen  rotundum,  165. 
Fore  brain,  48. 
Formicaria,  351. 
Fornix,  55. 

Fossa  rhomboidalis,  54. 
Fowl,  350. 
Fox,  412. 
Fregata,  347. 
Frigate  bird,  347. 


426 


INDEX. 


Frigate  mackerel,  263. 

Gastrosplenic  omentum,  106. 

Frilled  lizard,  320. 

Gastrula,  5. 

Frogs,  287. 

Gastrulation,  211. 

Frontal  bone,  161. 

Gavialis,  328. 

Frugivora,  388. 

Gavialidae,  328. 

Fulcra,  248. 

Gazella,  399. 

Fulmarus,  349. 

Gecko,  319. 

Fundulus,  257. 

Geese,  348. 

Fundus  glands,  367. 

Gemsbok,  399. 

Fundus  region,  35. 

General  cutaneous  nerves,64. 

Furcula,  336. 

Genital  artery,  191. 

Fur  seal,  413. 

Geococcyx,  348. 

Geomyidae,  390. 

Gadidae,  264. 

Geomys,  390. 

Gadus,  264. 

Geotria,  223. 

Galago,  415. 

Germ  layers,  8. 

Galeidae,  238. 

Gibbon,  416. 

Galeocerdo,  239. 

Gill  clefts,  22. 

Galeopethecidae,  415. 

Gill  rakers,  230. 

Galeopithecus,  385,  415. 

Gills,  22. 

Galerix,  385. 

Ginglymodi,  251. 

Galesaurus,  306. 

Giraffa,  399. 

Galeus,  239. 

Giraldi's  organ,  130. 

Gall  bladder,  40. 

Gizzard,  34. 

Gall  capillaries,  40. 

Gland,  acinose,  90. 

Gallinae,  350. 

"      dermal,  90. 

Gallinago,  350. 

"      Harder's,  84. 

Gallus,  350! 

"      internasal,  21. 

Ganglion,  n. 

"       lachrymal,  84. 

"        Casserian,  61. 

"      milk,  91. 

"        cells,  10.                            "      oil,  98. 

"        ciliary,  62. 

"      oral,  21, 

"        of  dorsal  roots,  46. 

"      parotid,  22. 

"    '    Gasserian,  61. 

"       racemose,  90. 

"        otic,  62. 

"       rectal,  36. 

"     '  sphenopalatine,  62. 

"       salivary,  22. 

Ganoidea,  248. 

"      sublingual,  22. 

Ganoid  scales,  228. 

"       submaxillary,  22. 

Ganoin,  92. 

"      sweat,  90. 

Ganhet,  347. 

"     lihymus,  33. 

Gar,  bony,  257. 

"      thyroid,  32. 

Gar  pikes,  251. 

"      tubular,  90. 

Garter  snake,  325. 

Glass-snake,  320. 

Gartner's  duct,  129. 

Glenoid  fossa,  168. 

Gasserian  ganglion,  61. 

Glia  cells,  12. 

Gasterosteidae,  258. 

Glires,  388. 

Gasterosteus,  258. 

Globe  fish,  267. 

Gastornis,  346. 

Globiocephalus,  408. 

Gastornithes,  346. 

Glomerulus,  119, 

Gastralia,  147. 

Glomus,  118. 

Gastric  artery,  190. 

Glossophaga,  388. 

Gastrostomus,  257. 

Glossopharyngeal  nerve,  63. 

Gastrohepa'ic  omentum,  105. 

Glottis,  27. 

Gastrolepidoti,  284. 

Glyptodon,  383. 

Glyptodontidae,  383. 
GnathostDmata,  225. 
Gnu,  399. 
Goat  fish,  261. 
Goats,  399. 
Gobiesocidae,  260. 
Gobiesox,  260. 
Gobioidia,  263. 
Gobius,  263. 
Gold  fish,  255. 
Golden  moles,  385. 
Golden  yellow  body,  303. 
Coil's  column,  54. 
Gonads,  124. 
Goniopholidae,  328. 
Goniopholis,  328. 
Gonotome,  103. 
Goose,  348. 
Goose  fish,  266. 
Gopher,  390. 
Gopher  turtle,  311. 
Gorilla,  416. 
Goura,  350. 
Gouramy,  262. 
Graafian  follicle,  124. 
Gradientia,  284. 
Grallae,  349. 

Grandry's  corpuscles,  68. 
Granular  layer,  79. 
Gray  matter,  n. 
Great  omentum,  106. 
Grebe,  349. 
Green  turtle,  310. 
Gronias,  255. 
Grouse,  350. 
Grus,  349. 
Guanaco,  398. 
Guillemots,  349. 
Guinea-pig,  391. 
Gull,  349. 
Gullet,  34. 
Gulo,  412. 
Gunnellus,  261. 
Gurnard,  260. 
Gymnodonti,  267. 
Gymnopoedes,  330. 
Gymnophiona,  287. 
Gymnbtus,  255. 
Gyps,  348. 
Gyri,  52. 

Habenulae,  52. 
Haddock,  264. 
Haddock,  Norway,  259. 


INDEX. 


427 


Hadrosaurus,  317. 
Haemal  arch,  138. 
Haemal  process,  138. 
Haemal  spine,  138. 
Haemapophysis,  138. 
Haemulon,  259. 
Hag-fish,  224. 
Hair,  97,  352. 
Hair  cells,  73. 
Hake,  264. 
Halcyon,  348. 
Halecomorphi,  252. 
Halibut,  265. 
Halicore,  405. 
Halicoridae,  405. 
Halitherium,  405. 
Hallux,  176. 

Hammer-head  shark,  239. 
Hamster,  390. 
Hapa]e,  415. 
Hapalidae,  415. 
Haploceras,  399. 
Haplodoci,  265. 
Haplodon,  390. 
Haplodontidae,  390. 
Haplodont,  365. 
Haplomi,  257. 
Harderian  glands,  84. 
Hares,  391. 
Harriotta,  242. 
Hatteria,  313. 
Haversian  canals,  14. 
Hawk,  348. 
Head  cavities,  in. 
Head  kidney,  116,  118. 
Head,  segmentation  of,  201. 
Heart,  178,  184. 
Heart  muscle,  12. 
Hedgehog,  385. 
Heliornis,  349. 
Helladotherium,  399. 
Hell-bender,  285. 
Heloderma,  321. 
Helodermidae,  321. 
Hemiazygos  vein,  196. 
Hemibranchii,  257. 
Hemichordia,  i. 
Hemipenes,  304. 
Hemispheres,  cerebral,  49. 
Hemispheres  of  cerebellum, 

54- 

Hemitripterus,  260. 
Hepatic  artery,  190. 
Hepatic  duct,  41. 


Hepatic  vein,  192. 
Hepato-enteric  duct,  41. 
Heptanchus,  238. 
Heptatrema,  224. 
Herodias,  348. 
Herodii,  348. 
Heron,  348. 
Herpestes,  412. 
Herring,  256. 
Hesperomys,  390. 
Hesperornis,  344. 
Heterocercal  fin,  229. 
Heterodon,  325. 
Heterodont,  20. 
Heteropygii,  257. 
Heterosomata,  264. 
Heterostraci,  224. 
Hexanchus,  238. 
Hindbrain,  49. 
Hindgut,  36. 
Hipparion,  395. 
Hippocampus,  258. 
Hippoglossus,  265. 
Hippopotamidae,  397. 
Hippopotamus,  398. 
Hipposiderus,  387. 
Histiophorus,  263. 
Histology,  9. 
Hogs,  397. 
Holacanthus,  262. 
Holconoti,  260. 
Holconotus,  260. 
Holoblastic  eggs, 210. 
Holocentridae,  261. 
Holocentrum,  261. 
Holocephali,  240. 
Holoptychius,  249. 
Holostei,  251. 
Homaeosaurus,  314. 
Hominidae,  416. 
Homo,  417. 
Homocercal  fin,  229. 
Homodont,  ao,  364. 
Homunculus,  416. 
Honeycomb,  35. 
Honey  guide,  348. 
Hooded  seal,  414. 
Hoofs,  99. 
Hoopoe,  348. 
Hoplophoneus,  412. 
Hoplophorus,  383. 
Horn,  99. 
Horn  bill,  348. 
Horned  pout,  255. 


Horned  toad,  320. 
Horses,  395. 
Horse  mackerel,  263. 
Howling  monkey,  416. 
Humerus,  176. 
Humming  birds,  350. 
Hump-back  whale,  410. 
Hyaena,  412. 
Hyaenarctus,  412. 
Hyaenidae,  412. 
Hyaenodontidae,  411. 
Hyaline  cartilage,  14. 
Hydatid,  128. 
Hydrophidae,  325. 
Hydropotes,  399. 
Hydrochoerus,  391. 
Hyla,  287. 
Hylerpeton,  283. 
Hylidae,  287. 
Hylobates,  416, 
Hylonomus,  283. 
Hyoid,  155. 
Hyoid  arch,  155. 
Hyoruandibula,  155. 
Hyomandibularis  nerve,  62. 
Hyomoschus,  398. 
Hyoplastron,  308. 
Hyopotamus,  397. 
Hypaxial  muscles,  109. 
Hyperoartia,  223. 
Hyperodapedon,  314. 
Hyperoodon,  409. 
Hyperotretia,  224. 
Hypoblast,6. 
Hypobranchial,  154. 
Hypocentrum,  136. 
Hypocone,  365. 
Hypogastric  artery,  183,  190. 
Hypogastric  vein,  194,  197. 
Hypogeophis,  288. 
Hypohyal,  155. 
Hypomere,  101. 
Hypoplastron,  308. 
Hypophysial  duct,  52. 
Hypophysis,  52. 
Hypsiprymnidae,  380. 
Hypsiprymnus,  380. 
Hypsirhopus,  316. 
Hyrachius,  395. 
Hyracodon,  395. 
Hyracoidea,  402. 
Hyracotherium,  395. 
Hyrax,  402. 
Hystricidae,  391. 


428 


INDEX. 


Hystricomorpha,  390. 
Hystrix,  391. 

Ibex,  400. 
Ibis,  348. 
Ichthyophis,  288. 
Ichthyopsida,  226. 
Ichthyopterygia,  312. 
Ichthyornis,  344. 
Ichthyosauria,  312. 
Ichthyosaurus,  313. 
Ichthyotomi,  237. 
Ictobius,  255. 
Ictopsidae,  385. 
Iguanidae,  319. 
Iguanodon, 317. 
Ileocolic  valve,  36. 
Iliac  artery,  190. 
Iliac  vein,  197. 
Ilium,  171. 
Impennes,  347. 
Implacentalia,  375. 
Impregnation,  5. 
Incisors,  364. 
Incus,  74, 159. 
Indeciduata,  374. 
Indicator,  348. 
Indris,  415. 
Ineducabilia,  361. 
Infraclavicle,  169. 
Infratemporal  fossa,  166. 
Infundibula,  30. 
Infundibulum,  52. 
Ingluvies,  34. 
Inia,  408. 
Iniomi,  256. 
Innominate  vein,  197. 
Insectivora,  384. 
Insertion  of  muscle,  112. 
Intercalary  cartilages,  234. 
Intercentrum,  137. 
Intercostal  arteries,  191. 
Intercostal  vein,  196. 
Interhyal,  244. 
Intermedium,  176. 
Internal  carotid,  183. 
Internasal  gland,  21. 
Interoperculum,  161. 
Interrenal,  131. 
Interspinalia,  230. 
Interspinous  ligament,  137. 
Intestinal  caecum,  39. 
Intestinalis  nerve,  64. 
Invagination,  5. 


Invertebrates,  i. 
Involuntary  muscle,  12. 
Ipnops,  256. 
Iris,  83. 

Ischiatic  artery,  191. 
Ischiromyidae,  390. 
Ischium,  171. 
Ischyodus,  242. 
Ischyrhiza,  257. 
Isectolophus,  395. 
Isinglass,  251. 
Isodectes,  305. 
Isolating  cells,  67. 
Isospondyli,  255. 
Isteus,  257. 
Iter,  50. 
Ivory,  19. 
lynx,  351. 

Ja9ana,  349. 
Jackal,  412. 
Jacobson,  organ  of,  77. 
Jacobson's  commissure,  63. 
Jaguar,  412. 
Jerboa,  390. 
Jugal,  163. 
Jugular  fins,  231. 
Jugular  foramen,  166. 
Jugular  vein,  183. 
Jumping  mice,  390. 
Jumping  shrews,  385. 
Jungle  fowl,  350. 

Kangaroo,  380. 
Kangaroo  rat,  380. 
Keraterpeton,  283. 
Keratobranchial,  154. 
Keratohyal,  155. 
Kidney,  122. 
Killer  whale,  408. 
Killifish,  257. 
Kingbird,  351. 
Kingfisher,  348. 
King  of  the  herrings,  242. 
Kinosternidae,  311. 
Kinosternon,  311. 
Kiwi,  346. 
Knee  pan,  360. 
Koala,  380. 
Kogia,  409. 
Kupffer's  vesicle,  254. 

Labial  cartilage,  156. 
Labridae,  260. 


Labrus,  260. 
Labyrinth,  7 1. 
Labyrinthici,  262. 
Labyrinthodon,  284. 
Labyrinthodonta,  283. 
Labyrinthodontidee,  284. 
Lacerta,  320. 
Lacertidae,  320. 
Lacertilia,  318. 
Lachrymal  bone,  161. 
Lachrymal  duct,  76. 
Lachrymal  gland,  84. 
Lactophrys,  267. 
Lacuna,  15. 
Laelaps,  316. 
Lagena,  71. 
Lagomorpha,  391. 
Lagomyidee,  391. 
Lagomys,  391. 
Lagostomus,  391. 
Lamella,  15. 
Lamina  cribrosa,  357. 
Lamina  terminalis,  50. 
Lamna,  239. 
Lamnidse,  239. 
Lampetra,  223. 
Lamprey  eels,  223. 
Laopteryx,  344. 
Laosaurus,  317. 
Lariosaurus,  306. 
Larus,  349. 
Larynx,  28. 
Laternlis  nerve,  64. 
Lateral  line,  68. 
Lateral  plate  zone,  101. 
Leather  back  tortoise,  310. 
Leather  turtle,  310. 
Lebias,  257. 
Lemming,  390. 
Lechriodonta,  285. 
Lemur,  415. 
Lemuridas,  415. 
Lemuroidea,  414. 
Lens,  8.1. 
Leopard,  412. 
Lepidocottus,  260. 
Lepidopus,  263. 
Lepidotus,  252. 
Lepidosauria,  317. 
Lepidosiren,  273. 
Lepidosteidae,  251, 
Lepidosteus,  251. 
Lepomis,  259. 
Leporidae,  391. 


INDEX. 


429 


Lepospondyli,  283. 

Lymnohyus,  396. 

Leptictidas,  411. 

Lymph,  16. 

Leptocardii,  i. 

Lymph  glands,  200. 

Leptocephalus,  257. 

Lymph  hearts,  199. 

Leptochcerus,  397. 

Lymph  system,  198. 

Leptolepis,  256. 

Lynx,  412. 

Leptomeryx,  398. 

Lyomeri,  257. 

Leptotragulus,  398. 

Lyre  bird,  351. 

Lepus,  391. 

Lestornis,  344. 

Macacus,  416. 

Leuciscus,  255. 

Macaque,  416. 

Leucocytes,  16. 

Machaerodus,  412. 

Leydig's  duct,  126 

Mackerel,  263. 

Limbs,  167. 

Macrauchenidre,  395. 

Limbs,  origin  of,  172. 

Macrochelys,  311. 

Lingualis  nerve,  63. 

Macromeres,  222. 

Liodon,  322. 

Macropetalichthys,  272. 

Lion,  412. 

Macropodidae,  380. 

Lion,  sea,  413. 

Macropus,  380. 

Liparis,  260. 

Macroscelidae,  385. 

Liver,  18,  40. 

Macrotherium,  396. 

Liver  islands,  40. 

Macrurus,  264. 

Lizards,  318. 

Macruridae,  264. 

Llama,  398. 

Macula  acustica,  71. 

Lobus  inferior,  53. 

Malacanthidae,  261. 

Loggerhead  turtle,  310. 

Malaclemmys,  311. 

Longirostres,  328. 

Malapterurus,  255. 

Loon,  349. 

Malar  bone,  163,  357. 

Loph,  365. 

Malleus,  74,  159. 

Lophiidae,  266. 

Malpighian  body,  119. 

Lophiomys,  390. 

Malpighian  layer,  89. 

Lophius,  266. 

Malthe,  266. 

Lophobranchii,  258. 

Malthidae,  266. 

Lophodon,  395. 

Mammalia,  352. 

Lophodont,  365. 

Mammoth,  401. 

Lopholatilus,  261. 

Man,  416. 

Lophopsetta,  265. 

Manatee,  405. 

Loricaria,  255. 

Manatherium,  405. 

Loricata,  259,  326,  383. 

Manatidae,  405. 

Loris,  415. 

Manatus,  405. 

Lota,  264. 

Mandibularis  nerve,  61. 

Loxodon,  401. 

Mandril],  416. 

Lumbar  arteries,  191., 

Manidae,  382. 

Lumbar  plexus,  48. 

Manis,  382. 

Lumbar  region,  142. 

Manta,  240. 

Lump  fish,  260. 

Mantle,  51. 

Lunare,  177. 

Manubrium,  149. 

Lung  fish,  267. 

Manus,  176. 

Lung  pipes,  31. 

Manyplies,  35. 

Lungs,  27. 

Marmoset,  415. 

Lupus,  412. 

Marsipobranchii,  219. 

Lutjanus,  259. 

Marsupial  bones,  171. 

Lutra,  412. 

Marsupialia,  378. 

Lycaon,  412. 

Marsupium,  378. 

Marsupium  (of  eye) ,  340. 
Marten,  412. 
Mastodon,  401. 
Mastodonsaurus,  284. 
Matrix,  14. 

Maturation  of  egg,  209. 
Maxillaris  nerve,  61. 
Maxillary,  162. 
Meatus,  auditory,  75. 
Meckel's  cartilage,  156. 
Mecodonta,  285. 
Mediastinum,  106. 
Medulla  oblongata,  50. 
Medullary  folds,  43. 
Medullary  plate,  43. 
Medullary  groove,  43. 
Medullary  sheath,  II. 
Medullated  fibres,  n. 
Megachiroptera,  388. 
Megaderma,  387, 
Megalobatrachus,  285. 
Megalonyx,  383. 
Megalosaurus,  316. 
Megalotriton,  286. 
Megamys,  391. 
Megapodius,  350. 
Megaptera,  410. 
Megatheriidae,  383. 
Megatherium,  383. 
Meissner's  corpuscles,  69. 
Melanerpeton,  283. 
Meleagris,  350. 
Meles,  412. 
Mellivora,  412. 
Merlucius,  264. 
Melursus,  412. 
Membrane  bone,  15,  164. 
Menhaden,  256. 
Menidia,  262. 
Meniscotheriidae,  393. 
Menobranchus,  285. 
Menopoma,  285. 
Menura,  351. 
Mephitis,  412. 
Merkel's  corpuscles,  68. 
Meroblastic  eggs,  210. 
Merops,  348. 
Merychius,  398. 
Mesencephalon,  50. 
Mesenchymatous  structures, 

132. 

Mesenchyme,  8. 
Mesenteric  artery,  190. 
Mesentery,  103. 


430 

Mesethmoid,  158. 
Mesoarium,  106. 
Mesoblast,  6. 
Mesocardium,  106, 180. 
Mesocolon,  105. 
Mesodactyla,  393. 
Mesoderm,  7. 
Mesogaster,  105. 
Mesohippus,  395. 
Mesomere,  101. 
Mesonephric  duct,  119. 
Mesonephros,  116,  118. 
Mesonychidse,  411. 
Mesonyx,  411. 
Mesoplodon,  409. 
Mesoptervgium,  175. 
Mesorchium,  106. 
Mesorectum,  105. 
Mesothelium,  8. 
Mesothelial  structures,  100. 
Mesovarium,  106,  126. 
Metacarpus,  176. 
Metacoele,  101,  103. 
Metacone,  365. 
Metaconule,  365. 
Metanephros,  116. 
Metapophysis,  141. 
Metapterygium,  175. 
Metatarsus,  176. 
Metazoa,  i. 
Metencephalon,  50. 
Miacidae,  411. 
Mice,  390. 

Microchiroptera,  387. 
Microconodon,  377. 
Microgadus,  264. 
Micromeres,222. 
Micropterus,  259. 
Microsauria,  283. 
Micropyle,  207. 
Midas,  415. 
Mid  brain,  48. 
Middle  zone,  101. 
Mid  gut,  36. 
Midshipman,  266. 
Milk  dentition,  20. 
Milk  glands,  91. 
Milk  line,  354. 
Mink,  412. 
Mixosaurus,  313. 
Moa,  346. 
Moccasin,  326. 
Mola,  267. 
Molars,  364. 


INDEX. 


Mole  rat,  396. 

Musk  deer,  399. 

Mole-shrew,  385. 

Musk  ox,  400. 

Moles,  385. 

Musk  rat,  390. 

Molossus,  388. 

Musk  turtle,  311. 

Momotus,  348 

Musophaga,  348. 

Monacanthus.  266. 

Mustela,  412. 

Monachus,  414. 

Mustelidae,  412. 

Monasa,  348. 

Mustelus,  239. 

Mongoose,  412. 

Mycetes,  416. 

Monimostylica,  300. 

Myelencephalon,  50. 

Monitor,  320. 

Myctodera,  285. 

Monkeys,  415. 

Myelin,  n. 

Monocondylia,  291. 

Myliobatidae,  240. 

Monodelphia,  381. 

Mylodon,  383. 

Monodon,  408. 

Myocardium,  180. 

Monophyodont,  366. 

Myocomma,  103. 

Monopneumonia,  272. 

Myocoele,  101. 

Monotremata,  376. 

Myodes,  390. 

Moonfish,  263 

Myogalidae,  385. 

Moose,  399. 

Myomorpha,  390. 

Mordacia,  223. 

Myopotamus,  391. 

Morgagni,  sinus  of,  29. 

Myoseptum,  103. 

Mormyridae,  255. 

Myotomes,  101,  108. 

Moropus,  396. 

Myotome  zone,  101. 

Mosasauridaa,  322. 

Myoxidae,  390. 

Mosasaurus,  322. 

Myoxus,  390. 

Moschus,  399. 

Myrmecobius,  379. 

Motmot,  348. 

Myrmecophaga,  383. 

Motor  nerves,  46. 

Myrmecophagidae,  383. 

Mouse,  390. 

Mystacoceti,  409. 

Mound-bird,  350. 

Myxine,  224. 

Mouth,  18. 

Myxinidas,  224. 

Mud  eel,  285. 

Myxinoidei,  224. 

Mud-minnow,  257. 

Myzontes,  219. 

Mud  puppy,  285. 

Mud  turtle,  311. 

Nails,  99. 

Mugilidae,  262. 

Naja,  325. 

Mullets,  262. 

Nandu,  345. 

Mullidae,  261. 

Naosaurus,  306. 

Mullus,  261. 

Nares,  76. 

Multipolar  nerve  cells,  10. 

Narwal,  408. 

Multituberculata,  377. 

Nasal  bone,  161. 

Mummichog,  257. 

Nasal  capsules,  153. 

Muntjacs,  399. 

Nasal  glands,  76. 

Muraana,  257. 

Nasua,  412. 

Muraenidaa,  257. 

Nates,  53. 

Muridae,  390. 

Naucrates,  263. 

Murry,  257. 

Naviculare,  177. 

Mus,  390. 

Necrolemur,  415. 

Muscle,  development  of,  108. 

Nectogale,  385. 

Muscle  plates,  109. 

Necturus,  285. 

Muscular  system,  107. 

Needle  fish,  257. 

Muscular  tissues,  12. 

Nemopteryx,  264. 

Muskalonge,  257. 

Neobalaena,  410. 

INDEX. 


431 


Neomeris,  408. 

Neurilemma,  n. 

Neomylodon,  383. 

Neuroglia,  12. 

Nephridia,  116. 

Neuromeres  of  brain,  49. 

Nephrostomes,  117. 

Neuropore,  44. 

Nephrotome,  103. 

Newt,  285. 

Nerve,  abducens,  61. 

Nictitating  membrane,  82. 

"      accessory   of    Willis, 

Night  hawk,  348. 

64. 

Noctilio,  388. 

"      afferent,  47. 

Nomarthra,  382. 

"     auditory,  63. 

Nondeciduata,  374. 

"      buccalis,  62. 

Non-elastic  tissue,  13. 

"      cranial,  58. 

Norway  haddock,  259. 

41      efferent,  47. 

Nothosaurus,  306. 

"      glossopharyngeal.  63. 

Notidanidae,  238. 

"      general  cutaneous,  64. 

Notochord,  17,  134. 

"      hyomandibularis,  62. 

Notochordal  sheath,  135. 

"      intestinalis,  64. 

Notodelphys,  287. 

lateralis,  64. 

Nototherium,  380. 

"      lingualis,  63. 

Nototrema,  287. 

"      mandibularis,  61. 

Notropis,  255. 

"      maxillaris,  61. 

Nycteris,  387. 

"      mixed,  59. 

"      motor,  46. 

Oblique   muscle  of  eye,  84, 

"      oculomotor,  61. 

114. 

"      olfactory,  60. 

Oblique   muscles   of   trunk, 

"      ophthalmicus,  61,  62. 

"3- 

"      optic,  60. 

Obturator  foramen,  170. 

"      palatine,  62. 

Occipital  bone,  356. 

"      patheticus,  59. 

Octodon,  391. 

"      pneumogastric,  64. 

Octodontidae,  391. 

"      post-trematic,  63. 

Oculomotor  nerve,  61. 

"      pretrematic,  63. 

Odontoblasts,  16,  19. 

"      roots  of,  46. 

Odontoceti,  408. 

"      sensory,  46. 

Odontoholcae,  344. 

"      somatic,  64. 

Odontoid  process,  143. 

"      spinal,  46. 

Odontormae,  344. 

"      spinal  accessory,  59. 

Odontornithes,  344. 

"      sympathetic,  47. 

CEsophagus,  34. 

"      trifacial,  59. 

Oil  bird,  348. 

"      trigeminal,  61. 

Oil  gland,  98. 

"      trochlearis,  61. 

Olecranon  process,  360. 

"      vagus,  63. 

Olfactory  lobe,  52. 

"      visceral,  64. 

"         nerve,  60. 

"      cells,  10. 

"         organ,  75. 

Nervous  tissue,  10. 

"         tract,  60. 

Nesodon,  402. 

Oligobunus,  412. 

Nesopithecidse,  415. 

Oligosoma,  320. 

Nesopithecus,  415. 

Omasum,  35. 

Neural  arch,  138. 

Omentum,  105,  106. 

"       crest,  47. 

gastro-hepatic,  41. 

"       plate,  43. 

Omosternum,  148. 

process,  135. 

Omphalomesaraic  vein,  180, 

"       spine,  138. 

192. 

Neurapophysis,  135. 

Oncorhynchus,  256. 

Onychodus,  249. 
Operculum,  23,  161. 
Ophiderpeton,  283. 
Ophidia,  322. 
Ophidiidae,  261. 
Ophidium,  261. 
Ophidioida,  261. 
Ophiocephalidae,  262. 
Ophisaurus,  320. 
Ophthalmicus  nerve,  61,  62. 
Opisthocoelous,  139. 
Opisthocomi,  349. 
Opisthoglypha,  325. 
Opisthomyzon,  263. 
Opisthotic,  158. 
Opoderodonta,  326. 
Opossums,  379. 
Optic  chiasma,  61. 
Optic  lobes,  50. 
Optic  nerves,  60. 
Optic  stalk,  78. 
Optic  thalami,  49. 
Optic  tract,  61. 
Optic  vesicle,  78. 
Orang-utan,  416. 
Orbitosphenoid,  158. 
Orca,  408. 
Oreodon,  398. 
Oreodontidce,  398.. 
Organ  of  Corti,  73. 
Organ  of  Giraldi,  130.. 
Organ  of  Jacobson,  77.. 
Origin  of  muscle,  112. 
Ornithopoda,  317. 
Ornithodelphia,  376. 
Ornithorhynchidae,  376. 
Ornithorhynchus,  376. 
Ornithosauria,  329. 
Oronasal  groove,  76. 
Orthagoriscus,  267. 
Orthopoda,  316. 
Orycteropodidae,  382. 
Orycteropus,  382. 
Oryx,  399. 
Os  en  ceinture,  278. 
Os  entoglossum,  161. 
Os  lenticulare,  159^ 
Os  magnum,  177* 
Os  orbiculare,  159. 
Os  trans versum,  163-.. 
Oscines,  351. 
Osmerus,  256. 
Osphromenus,  262. 
Ossicula  auditus,  158^ 


432 


INDEX. 


Ossification,  14. 
Ossification,      perichondral, 

133. 

Ostariophysi,  254. 
Osteoblasts,  15,  133. 
Osteolepis,  250., 
Osteostraci,  225. 
Ostium  tubae.  127. 
Ostracion,  267. 
Ostracodermi,  224,  266. 
Ostrich,  345. 
Otaria,  413. 
Otariidae,  413. 
Otic  capsule,  151. 
Otic  ganglion,  62. 
Otic  vesicle,  70. 
Otis,  349. 
Otoccelus,  310. 
Otocyon,  412. 
Otoliths,  71. 
Otter,  412. 
Oudenodon,  305. 
Ova,  125. 

Ovarian  artery,  191. 
Ovaries,  125. 
Ovibos,  400. 
Oviduct,  127,  247. 
Ovis,  400. 

Ovum,  history  of,  205. 
Owls,  348. 
Oxyclaenidae,  411. 

Paca,  391.        , 
Pachycormus,  252. 
Pachylemuridae,  415. 
Pacini's  corpuscles,  69. 
Paddlefish,  251. 
Palaeobatrachidae,  287. 
Palaeeudyptes,  347. 
Palasonictidas,  411. 
Palaeonictis,  411. 
Palaeohatteria,  314. 
Palseoniscidae,  251. 
Palaeoniscus,  251. 
Palseorhynchidae,  263. 
Palaeotherium,  395. 
Paloeosyops,  396. 
Palate,  358. 
Palatine  bone,  163. 
Palatine  nerve,  62. 
Palatoquadrate,  156. 
Palinurichthys,  263. 
Pallium,  51. 
Palorchestes,  380. 


Pancreas,  41. 
Pangolin,  382. 
Panniculus  adiposus,  354. 
Panniculus  carnosus,  115. 
Panther,  412. 
Pantolambda,  400. 
Pantolestidae,  396. 
Parabronchi,  31. 
Parachordals,  151. 
Paracone,  365. 
Paraconid,  365. 
Paradidymis,  130. 
Paradisea,  350. 
Paralichthys,  265. 
Paraphysis,  87. 
Parapinealis,  86. 
Parapophysis,  141. 
Parasuchia,  328. 
Parasphenoid,  163. 
Pareiasauria,  304. 
Pareiasaurus,  304. 
Parietal  bone,  161. 
Parietal  eye,  85. 
Paroccipital,  161. 
Parocthus,  383. 
Parovarium,  129. 
Parrot  fish,  260. 
Parrots,  349. 
Partridge,  350. 
Passer,  351. 
Passeres,  351. 
Patella,  360. 
Patheticus  nerve,  59. 
Patriofelis,  411. 
Paunch,  35. 

Pavement  epithelium,  9. 
Pavo,  350. 
Peafowl,  350. 
Peccaries,  397. 
Pecora,  396. 
Pecten,  340. 
Pecten  of  eye,  296. 
Pectineal  process,  336. 
Pectoral  fins,  231. 
Pectoral  girdle,  168. 
Pectoral  limb,  167. 
Pedetes,  390. 
Pediculati,  266. 
Peduncles  of  brain,  54. 
Pegasus,  260. 
Pelecanus,  347. 
Pelican,  347. 
Pelobates,  286. 
Pelobatidse,  286. 


Pelomedusa,  311. 
Peltephilus,  384. 
Pelvic  girdle,  168. 
Pelvic  limb,  167 . 
Pelvis  renalis,  123. 
Pelycodus,  415. 
Pelycosauria,  306. 
Penguin,  347. 
Perameles,  380. 
Peramelidae,  379. 
Perca,  259. 
Percesoces,  262. 
Perch,  259. 
Perch,  surf,  260. 
Percidae,  259. 
Percoidea,  259. 
Percopsis,  259. 
Perdix,  350. 

Perennibranchiata,  284. 
Pericardio-peritoneal  canals, 

106. 

Pericardium,  106,  179. 
Perichondral       ossification, 

133- 

Perichondrium,  15. 
Perilymph,  72. 
Perimysium,  12,  112. 
Perineum,  130. 
Periosteum,  15. 
Peritoneal  cavity,  106. 
Peritoneal  layer,  39. 
Peritoneum,  103. 
Periptychidae,  393. 
Permanent  dentition,  20. 
Peropoda,  325. 
Peroneal  artery,  191. 
Petalopteryx,  259. 
Petaurus,  380. 
Petrel,  349. 
Petromyzon,  223. 
Petromyzontes,  223. 
Petrosal,  166,  356. 
Phacochcerus,  397. 
Phaethon,  347. 
Phalacrocorax,  347. 
Phalanges,  176. 
Phalangista,  380. 
Phalangistidoe,  380. 
Phanerobranchia,  284. 
Phaneropleuron,  273. 
Pharyngeal  bones,  244. 
Pharyngobranchial,  154. 
Pharyngognathi,  260. 
Phascalonus,  380. 


INDEX. 


433 


Phascogale,  379. 
Phascolarctos,  380. 
Phascolomyidae,  380. 
Phascolomys,  380. 
Phasianus,  350. 
Pheasant,  350. 
Phenacodidae,  393. 
Phenicopterus,  348. 
Phlegethontia,  283. 
Phoca,  414. 
Phocaena,  408. 
Phocidae,  413. 
Phrynosoma,  320. 
Phycis,  264. 
Phyllodactylus,  319. 
Phyllospondylous,  137. 
Phyllostomidae,  388. 
Physeter,  409. 
Physeteridae,  409. 
Physoclisti,  25,  254. 
Physostomi,  255. 
Pia  mater,  57. 
Pica,  391. 
Picariae,  350. 
Pickerel,  257. 
Picus,  351. 
Pig,  guinea,  391. 
Pigeon,  350. 

Pigs,  397- 
Pike,  257. 
Pillar  cells,  73. 
Pilot  fish,  263. 
Pineal  eye,  85. 
Pinnipedia,  413. 
Pipa,  286. 
Pipe  fish,  258. 
Pirate  perch,  259. 
Pisces,  227. 
Pisiforme,  177. 
Pituitary  body,  52.. 
Placenta,  290,  373. 
Placentalia,  375,  381. 
Placodontia,  306. 
Placodus,  306. 
Placoid  scales,  92. 
Plagiaulax,  378. 
Plagiostomi,  232. 
Plagiotremata,  317. 
Plaice,  265. 
Plantain  eater,  348. 
Plantigrade,  361. 
Plastron,  93,  308. 
Platanista,  408. 
Platalea,  348. 


Platanistidae,  408. 
Platax,  263. 
Platecarpus,  322. 
Platessa,  265. 
Platycephalus,  260. 
Platycormus,  261. 
Platydactylus,  319. 
Platygonus,  397. 
Platyops,  284. 
Platyrhini,  415. 
Platysomus,  251. 
Plecotus,  387. 
Plectognathi,  266. 
Plesiosauria,  306. 
Plesiosaurus,  306. 
Plethodon,  285. 
Pleuracanthus,  237. 
Pleural  cavity,  106. 
Pleural  layer,  8. 
Pleurapophysis,  141. 
Pleuraspidotheriidae,  393. 
Pleurocentrum,  136. 
Pleurodeles,  286. 
Pleurodira,3ii. 
Pleurodont  teeth,  294. 
Pleuronectes,  265. 
Pleuronectidae,  265. 
Pleuropterygii,  237. 
Plexus,  choroid,  52. 
Plexus,  nerve,  48. 
Plica  semilunaris,  83. 
Pliauchenia,  398. 
Plioplatecarpidae,  322. 
Plioplatecarpus,  322. 
Pliosaurus,  307. 
Ploughshare  bone,  331. 
Plover,  349. 
Pneumatic  duct,  25. 
Pneumatocyst,  25. 
Pneumogastric  nerve,  63,  64. 
Pocket  gopher,  390. 
Podiceps,  349. 
Podocnemis,  311. 
Poebrotherium,  398. 
Poison  teeth,  324. 
Polar  globule,  209. 
Polistotrema,  224. 
Pollachius,  264. 
Pollack,  264. 
Pollex,  176. 
Polydactylus,  262. 
Polymastodon,  378. 
Polynemidse,  262. 
Polyodon,  251. 


Polyodontidae,  251. 
Polyonax,  317. 
Polyprotodontia,  379. 
Polypteridae,  250. 
Polypterus,  250. 
Pomacanthus,  262. 
Pomacentridae,  261. 
Pomatomidae,  263. 
Pompano,  263. 
Popliteal  artery,  191. 
Porcupine,  391. 
Porcus,  397. 
Pori  abdominales,  107. 
Porichthys,  266. 
Porpoise,  408. 
Portal  system,  192. 
Portal  vein,  193. 
Portheus,  256. 
Postcardinal  vein,  183,  193. 
Postcava,  195. 
Postclavicle,  169. 
Post  frontal,  161. 
Postorbital,  161. 
Post  temporal,  169,  245. 
Post  trematic  nerve,  63. 
Postzygopophysis,  140. 
Prairie  dog,  390. 
Pratincole,  349. 
Precava,  197. 
Precoces,  330. 
Prefrontal,  161,  358. 
Premaxillary,  161. 
Premolars,  366. 
Preoperculum,  161. 
Presphenoid,  158. 
Presternum,  356. 
Pretrematic  nerves,  63. 
Prezygapophysis,  140. 
Priacanthus,  259. 
Primaries,  330. 
Primates,  414. 
Primitive  streak,  7,  342. 
Primitive  groove,  7. 
Primordial  cranium,  150. 
Prionotus,  260. 
Pristidae,  239. 
Pristiophoridae,  239. 
Pristis,  239. 
Proatlas,  143. 
Proboscidea,  400. 
Procamelus,  398. 
Procartilage,  133. 
Procellaria,  349. 
Processus  falciformis,  231. 


434 


INDEX. 


Procoelous,  139. 

Pterosauria,  329. 

Procoracoid,  170. 

Pterotic,  244. 

Procyon,  412. 

Pterygoid,  158. 

Procyonidae,  412. 

Pterygoid  process,  356. 

Proganochelys,  311. 

Pterygoquadrate,  156. 

Pronephric  duct,  117. 

Pterylae,  95. 

Pronephros,  116. 

Ptychodrilus,  255. 

Prong-horn,  399. 

Ptychozoon,  319. 

Prootic,  158. 

Ptyctodontidae,  242. 

Prorastomidae,  405. 

Ptyctodus,  242. 

Prorastomus,  405. 

Pubis,  171. 

Prosencephalon,  49. 

Puff-bird,  348. 

Prosimise,  414. 

Pullastrae,  350. 

Prostate  glands,  371. 

Pulmonary  artery,  185. 

Proteidae,  285. 

Pulp  cavity,  19. 

Proteles,  412. 

Puma,  412. 

Proteroglypha,  325. 

Pupil,  83. 

Proterosaurus,  314. 

Pygostyle,  142,  331. 

Proteus,  285. 

Pyloric  appendages,  38. 

Protoceras,  399. 

Pyloric  gland,  367. 

Protocone,  365. 

Pyloric  stomach,  337. 

Protoconule,  365. 

Pylorus,  34. 

Protodonta,  377. 

Pyramidalis  muscle,  340. 

Protohippus,  395. 

Python,  325. 

Protolabis,  398. 

Pythonomorpha,  321. 

Protopterus,  273. 

Protopterygium,  175. 

Quadrate,  158. 

Protoreodon,  398. 

Quadratus  muscle,  340. 

Protostega,  310. 

Quail,  350. 

Prototheria,  376. 

Prototheridae,  395. 

Rabbits,  391. 

Protovertebrae,  101. 

Raccoon,  412. 

Proventriculus,  34. 

Raccoon  fox,  412. 

Provivera,  411. 

Racemose  glands,  90. 

Psalterium,  35. 

Rachiodon,  325. 

Psephoderma,  310. 

Radial  artery,  189. 

Psephurus,  251. 

Radiale,  176. 

Psetta,  265. 

Radialia,  173. 

Psittaci,  349. 

Radius,  176. 

Pseudob  ranch,  23. 

Radix  aortae,  182. 

Pseudopleuronectes,  265. 

Raiae,  239. 

Pseudosuchia,  327. 

Rail,  349. 

Pseudo  ventricle,  57. 

Raja,  239. 

Pteranodon,  330. 

Rajidae,  239. 

Pteraspis,  224. 

Rallus,  349. 

Pterichthys,  225. 

Ram  us  dorsalis,  47. 

Pterocletes  ,  350. 

Ramus  intestinalis,  47. 

Pterodactylia,  329. 

Ramus  ventralis,  47. 

Pterodactylidae,  330. 

Rana,  287. 

Pteromys,  390. 

Ranidae,  287. 

Pteropaedes,  330. 

Rangifer,  399. 

Pterophryne,  266. 

Raptores,  348. 

Pteropodidae,  388. 

Rasores,  350. 

Pteropus,  388. 

Ratitae,  149, 

Rattlesnake,  325. 

Rats,  390. 

Rectal  gland,  36. 

Rectrices,  330. 

Rectum,  38. 

Rectus  muscle    of  eye,   84, 

114. 
Rectus    muscles    of   trunk, 

"3- 

Reduction  division,  208. 
Regalecidae,  264. 
Regalecus,  264. 
Reindeer,  399. 
Remiges,330. 
Remora,  264. 
Renal  artery,  191. 
Renal  portal  system,  196. 
Renal  vein,  196. 
Reproductive  organs,  124. 
Rennet,  35. 
Respiratory  tract,  76. 
Rete,  130. 

Reticulate  tarsus,  331. 
Reticulum,  35. 
Retractor  bulbi,  84. 
Rhachianectes,  410. 
Rhachitomi,  283. 
Rhachitomous,  136. 
Rhabdopleura,  3. 
Rhamphastos,  348. 
Rhamphostoma,  328. 
Rhea,  346, 
Rheidse,  345. 
Rhinencephalon,  52. 
Rhinoceridae,  395. 
Rhinoceros,  395. 
Rhinolophidas,  387. 
Rhinolophus,  387. 
Rhinophis,  326. 
Rhinotrema,  288. 
Rhipidistia,  249. 
Rhombodipterini,  249. 
Rhombus,  263,  265. 
Rhynchocephalia,  313. 
Rhynchodus,  242. 
Rhynchosuchus,  328. 
Rhynchotus,  346. 
Rhytina,  405. 
Rhytinidae,  405. 
Rhytiodus,  405. 
Rib,  143. 
Ribbon  fish,  264. 
Ribodon,  405. 
Right  whale,  410. 


INDEX. 


435 


Rockfish,  259. 

Scaphiopus,  286. 

Rod  cells,  79. 

Scaphirhynchus,  251. 

Rodentia,  388. 

Scaphoid,  177. 

Roller,  348. 

Scapula,  168. 

Rorqual,  410. 

Scarus,  260. 

Rostral  bone,  299,  316. 

Sceleporus,  320. 

Rostrum  (birds),  334. 

Scelidosaurus,  316. 

Rudder  fish,  263. 

Scelotes,32o. 

Rumen,  35. 

Schizocoele,  7. 

Ruminantia,  396. 

Schizognathi,  349. 

Rumination,  396. 

Schizognathous,  335. 

Rupicapra,  399. 

Schwann,  sheath  of,  11. 

Sciaenidae,  259. 

Sable,  412. 

Sciatic  artery,  191. 

Saccomys,  390. 

Scincidae,  320. 

Sacculus,  70. 

Scincus,  320. 

Sacculus    endolymphaticus, 

Sciuridae,  390. 

70. 

Sciuromorpha,  389. 

Sacculus  vasculosus,  53. 

Sciuropterus,  390. 

Sacral  plexus,  48. 

Sciurus,  390. 

Sacrum,  141. 

Sclerodermi,  266. 

Sagenodus,  273. 

Sclerotic,  83,  153. 

Saiga,  399. 

Sclerotomes,  102. 

Sail  fish,  263. 

Scolopax,  349. 

Salamanders,  285. 

Scomber,  263. 

Salamandra,  286. 

Scomberesox,  257. 

Salamandrina,  285. 

Scombridae,  263. 

Salientia,  286. 

Scombroidae,  263. 

Salivary  glands,  21. 

Scopelus,  256. 

Salmo,  256. 

Scops,  348. 

Salmon,  256. 

Scorpaena,  259. 

Salmonidae,  256. 

Scorpaenidae,  259. 

Salmopercae,  259. 

Scotasops,  383. 

Samotherium,  399. 

Screamer,  348. 

Sand-grouse,  350. 

Scrotum,  371. 

Sand  launce,  263. 

Sculpin,  259. 

Sapajou,  416. 

Scutellate  tarsus,  331. 

Sarcolemma,  12. 

Scutes,  322. 

Sarcorhamphus,  348. 

Sea  bats,  266. 

Sargus,  259. 

Sea  cow,  405. 

Saurii,  378. 

Sea  horse,  258. 

Saurocephalus,  256. 

Sea  lion,  413. 

Saurognathous,  335. 

Sea  otter,  412. 

Sauropoda,  315. 

Sea  robin,  260. 

Sauropsida,  291. 

Sea  snake,  325. 

Sauropterygii,  306. 

Seals,  413. 

Saurorhamphus,  258. 

Sebastes,  259. 

Saururae,  343. 

Secodont,  365. 

Saw  fish,  239. 

Secondaries,  330. 

Savi's  vesicles,  68. 

Sectorial  teeth,  411. 

Scalse,  73. 

Segmentation  cavity,  5,  210. 

Scales,  92,  99,  228. 

Segmentation  of  egg,  5,  209. 

Scalops,  385. 

Segmentation  of  head,  201. 

Scaly  ant-eater,  382. 

Segmentation  nucleus,  209. 

Selachii,  238. 
Selachostomi,  251. 
Selene,  263. 
Selenodont,  366. 
Semicircular  canals,  70. 
Semnopithecus,  416. 
Sense  capsules,  66. 
Sense  cells,  66. 
Sense  corpuscles,  68. 
Sense  organs,  66. 
Sensory  nerves,  46. 
Seps,  320. 

Septum  pellucidum,  51. 
Septum  transversum,  106. 
Seriola,  263. 
Serpentes,  322. 
Serranidae,  259. 
Sesamoid  bones,  176. 
Seven-sleeper,  390. 
Sewellel,  390. 
Shad,  256. 
Sharks,  232,  238. 
Sheath  of  Schwann,  n. 
Sheep,  399. 
Shoulder  girdle,  168. 
Shovel  nose,  251. 
Shrews,  385. 
Siluridae,  255. 
Silversides,  262. 
Simaedosaurus,  314. 
Simia,  416. 
Simiae,  415. 
Simiidae,  416. 
Sinus  of  Morgagni,  29.  . 
Sinus  urogenitalis,  130. 
Sinus  venosus,  181. 
Siphoneum,  340. 
Siredon,  285. 
Siren,  285. 
Sirenia,  403. 
Sirenidae,  284. 
Sirenoidea,  272. 
Sivatherium,  399. 
Skates,  232. 
Skink,  320. 
Skua,  349. 
Skull,  150. 
Skunk,  412. 
Sloths,  383. 
Small  omentum,  105. 
Smell,  organ  of,  75- 
Smelt,  256. 
Smooth  muscle,  12. 
Snakes,  322. 


436 


INDEX. 


Snapping  turtle,  311. 
Snipe,  349. 
Sole,  265. 
Solea,  265. 
Soleidae,  265. 
Solenodontidae,  385. 
Solenoglypha,  325. 
Solenorhynchus,  258. 
Solenostoma,  258. 
Solenostomidae,  258. 
Solidungula,  393. 
Somatic  layer,  8. 
Somatic  nerves,  64. 
Somites,  100. 
Sorex,  385. 
Soricidae,  385 
Spade  foot  toad,  287. 
Spalax,  390. 
Sparidae,  259. 
Sparrow,  351. 
Spelerpes,  285. 
Spermatic  artery,  191. 
Spermatophores,  209. 
Spermatozoon,  5,  207. 
Spermophilus,  390. 
Sperm  whale,  409. 
Sphargis,  310. 
Sphenethmoid,  278. 
Spheniscus,  347. 
Sphenodon,  314. 
Sphenodontina,  314. 
Sphenoid,  356. 
Sphenoidal  fissure,  164. 
Sphenopalatine  ganglion,  62. 
Sphenotic,  244. 
Sphrynidae,  239. 
Sphyraena,  262. 
Sphyraenidae,  262. 
Spinal  accessory  nerve,  59. 
Spinal  cord,  44. 
Spinal  nerves,  46. 
Spinous  process,  138. 
Spiny  ant-eater,  377. 
Spiracle,  23,  230. 
Spiral  valve,  36. 
Splanchnic  layer,  8. 
Splanchnocoele,  101,  103. 
Spleen,  200. 
Splenial  bone,  164. 
Splenic  artery,  190. 
Splint  bone,  394. 
Spoonbill,  348. 
Spreading  viper,  325. 
Springbok,  399. 


Squalidae,  239. 

Sub-mucosa,  39. 

Squalodontidae,  408. 

Sub-operculum,  161. 

Squaloraia,  242. 

Subungulata,  392. 

Squalus,  239. 

Suckfish,  264. 

Squamata,  317,  382. 

Suckers,  255. 

Squamipinnes,  262. 

Suidae,  397. 

Squamosal,  161. 

Suina,  396. 

Squatinidae,  239. 

Sula,  347. 

Squirrel,  390. 

Sunfish,  259,  267. 

Stagodon,  403. 

Sun  grebe,  349. 

Stapes,  74,  159. 

Supra-angulare,  164. 

Star  gazer,  261. 

Supraclavicle,  169. 

Steatornis,  348. 

Suprascapula,  168. 

Steatornithidae,  348. 

Supraoccipital,  158. 

Steganopodes,  347. 

Supraorbital,  161. 

Stegocephali,  283. 

Suprarenal  body,  131. 

Stegodon,  401. 

Supratemporal  fossa,  166. 

Stegosauria,  316. 

Surf  perch,  260. 

Stegosaurus,  316. 

Surinam  toad,  286. 

Stenofiber,  390. 

Surmullet,  261. 

Stenostoma,  326. 

Suspensorium,  155. 

Stenson's  duct,  77. 

Sus,  397. 

Stercorarius,  349. 

Swan,  348. 

Stereospondyli,  284. 

Sweat  glands,  90. 

Sterna,  349. 

Swell  fish,  267. 

Sternebrae,  148. 

Swift,  320. 

Sternothasrus,  311. 

Swimbladder,  25. 

Sternum,  147. 

Swine,  397. 

Stickleback,  258. 

Swordfish,  263. 

Sting  ray,  239. 

Sympathetic  system,  47. 

Stomach,  34. 

Syncitium,  12. 

Stomodeum,  18. 

Synentognathi,  257. 

Stork,  348. 

Synetheres,  391  . 

Strand  rat,  390. 

Syngnathus,  258. 

Stratified  epithelium,  9. 

Syngnathidae,  258. 

Stratiodontidae,  257. 

Synotic  tectum,  151. 

Stratum  corneura,  88. 

Synsacrum,  331. 

Stratum  lucidum,  89. 

Syphostoma,  258. 

Streptostylica,  300. 

Syrinx,  29. 

Striped  muscle,  12. 

Strix,  348. 

Tactile  cells,  68. 

Stromateidae,  263. 

Taeniodonta,  402. 

Struthio,  345. 

Taeniosomi,  264. 

Struthiones,  345. 

Tail  coverts,  330. 

Struthionidae,345. 

Talon,  365. 

Sturgeon,  250. 

Talpa,  385. 

Style,  365. 

Talpidae,  385. 

Stylephorus,  264. 

Tamias,  390. 

Stylinodon,  403. 

Tapetum,  863. 

Stylohyoid,  165. 

Tapiridae,  395. 

Subclavian  artery,  188. 

Tapir  us,  395. 

Subclavian  vein,  194,  197. 

Tardigrada,  383. 

Subintestinal  vein,  193. 

Tarsiidae,  415. 

Sublingua,  21,  364. 

Tarsipes,  380. 

INDEX. 


437 


Tarsius,  415. 
Tarso-metatarsus,  336. 
Tarsus,  176. 
Taste,  68. 
Tatusia,  383. 
Tautoga,  260. 
Taxeopoda,  393. 
Taxidea,  412. 
Teeth,  19. 
Tegmen  cranii,  152. 
Tegumentary  skeleton,  91. 
TeidcE,  321. 
Tejus,  321. 
Telencephalon,  49. 
Teleostei,  252. 
Teleostomi,  242. 
Telerpeton,  314. 
Telolecithal  eggs,  206. 
Telosaurus,  328. 
Temnocyon,  412. 
Temnospondyli,  283. 
Temporal  bone,  161. 
Temporal  fossa,  167. 
Tendons,  112. 
Tenrec,  385. 
Tern,  349. 
Terrapin,  311. 
Tertiaries,  330. 
Testes,  126. 
Testes  of  brain,  53. 
Testudinata,  307. 
Testudinidae,  311. 
Testudo,  311. 
Tetrao,  350. 
Tetraprotodon,  398. 
Tetrodon,  267. 
Teuthidae,  262. 
Teuthis,  262. 
Thalamencephalon,  49. 
Thalami,  49,  52. 
Thalassochelys,  310. 
Thalassophryne,  266. 
Thaumalea,  350. 
Thecadont  teeth,  294,  364. 
Theriodontia,  306. 
Theromorpha,  304. 
Theropoda,  316. 
Thomomys,  390. 
Thoracic  duct,  199. 
Thoracic  fins,  231. 
Thoracic  region,  142. 
Thoracosaurus,  328. 
Thread  cells,  220. 
Thresher  sharks,  239. 


Thylacinus,  379. 
Thylacoleo,  378. 
Thylacoleonidas,  380. 
Thymus  glands,  33. 
Thynnus,  263. 
Thyrohyoid,  165. 
Thyroid  cartilage,  28. 
Thyroid  gland,  32. 
Thyroptera,  387.' 
Tibia,  176. 
Tibial  artery,  191. 
Tibiale,  176. 
Tibio-tarsus,  336. 
Ticholeptus,  398. 
Tiger,  412. 
Tiger  sharks,  239. 
Tile  fish,  261. 
Tillodontia,  402. 
Tillotherium,  403. 
Tinamus,  346. 
Tissues,  9. 
Titanichthys,  272. 
Titanotheriidae,  396. 
Titanotherium,  396. 
Toad  fish,  266. 
Toad,  horned,  320. 
Toads,  286. 
Todus,  348. 
Tody,  348. 
Tolypeutes,  383. 
Tomcod,  264. 
Tomitherium,  415. 
Tongue,  21. 
Tonsils,  200. 
Toothed  birds,  344. 
Torpedinidas,  239. 
Torpedo,  239. 
Tortoise,  310. 
Tortoise  shell,  308. 
Tortoise  shell  turtle,  310. 
Tortricina,  326. 
Tortrix,  326. 
Toucan,  348. 
Toxodon,  262,  402. 
Toxodontia,  402. 
Trabeculae  cranii,  151. 
Trachea,  27. 
Trachinidae,  261. 
Trachinoidae,  261. 
Trachinus,  261. 
Trachynotus,  263. 
Trachypteridas,  264. 
Trachypterus,  264. 
Tragelaphus,  399. 


Tragulidae,  398. 
Tragulina,  396. 
Tragulus,  398. 
Transverse  process,  141. 
Trapezium,  177. 
Trapezoid,  177. 
Tree  kangaroo ,  380. 
Tree  toads,  287. 
Trematosaurus,  284. 
Triceratops,  317. 
Trichechidae,  413. 
Trichechus,  413. 
Trichecus,  405. 
Trichiuridae,  263. 
Trichiurus,  263. 
Trichoglossus,  349. 
Triconodont,  365. 
Triconodonta,  380. 
Trifacial  nerve,  59. 
Trigeminal  nerve,  61. 
Trigger  fish,  266. 
Trigla,  260. 
Triglidae,  260. 
Trigon,  365. 
Trimerorhachis,  283. 
Triton,  285. 
Tritors,  241. 
Tritubercular,  365. 
Trituberculata,  380. 
Trochanter,  360. 
Trochilidae,  350. 
Trochlearis  nerve,  61. 
Troglodytes,  416. 
Trogonidae,  348. 
Tropic  birds,  347. 
Tropidonotus,  325. 
Tropidosaurus,  320. 
Trout,  256. 

Truncus  arteriosus^  181. 
Trunk  fish,  267. 
Trygonidas,  239. 
Tryonychia,  310. 
Tuber  cinereum,  53. 
Tubinares,  349. 
Tubular  glands,  90. 
Tunicata,  I. 
Tunny,  263. 
Tupaia,  385. 
Tupaiidae,  385. 
Turbinal  bones,  75,  338. 
Turbot,  265. 
Turkey,  350. 
Tursiops,  408. 
Turtles,  310. 


438 


.  INDEX. 


Tutidanus,  283. 
Twixt  brain,  49. 
Tylopoda,  396,  398. 
Tylosurus,  257. 
Tympanic  bone,  357. 
Tympanum,  73. 
Typhline,  320. 
Typhlogobius,  263. 
Typhlonectes,  288. 
Typhlops,  326. 
Typotherium,  402. 
Typothrax,  328. 
Tyrannus,  351. 
Tyrant  bird,  351. 

Uintatherium,  400. 
Ulna,  176. 
Ulnar  artery,  189. 
Ulnare,  176. 
Umbilicus,  288. 
Umbra,  257. 
Umbridae,  257. 
Unciforme,  177. 
Uncinate  process,  146. 
Ungulata,  391. 
Ungulata  Vera,  392. 
Unguligrade,  361. 
Unicorn  fish,  266. 
Unipolar  nerve  cells,  '10. 
Upeneus,  261. 
Upupa,  348. 
Ur,  400. 
Uranidea,  259. 
Uranoscopus,  261. 
Ureter,  122. 
Urethra,  124. 
Uria,  349. 
Urinator,  349. 
Urochordia,  i. 
Urodela,  284. 
Urogenital  ducts,  126. 
Urogenital  organs,  116. 
Urogenital  sinus,  130. 
Urohyal,  335. 
Uropeltes,  326. 
Urostyle,  142. 
Urotrichus,  385. 
Ursidae,  412. 
Ursus,  412. 
Uterine  placenta,  374. 
Uterus,  127. 

Uterus  masculinus,  128. 
Utriculus,  70. 


Vagina,  127. 

Viveridae,  412. 

Vagus  nerve,  63. 

Vizcacha,  391. 

Valve,  ileocolic,  36. 

Vocal  cords,  29. 

Valve,  spiral,  36. 

Vole,  390. 

Valve  of  Vieussens,  54. 

Voluntary  muscle,  12. 

Valvulae  conniventes,  38. 

Vomer,  163. 

Vampyre  bat,  388. 

Vomer  (fish),  263. 

Vampyrus,  388. 

Vulpes,  412. 

Varanidae,  320. 

Vultures,  348. 

Varanus,  320. 

Vas  aberrans,  130. 

Vas  deferens,  130. 

Wagner's  corpuscles,  69. 

Vas  efferens,  130. 

Wallaby,  380. 

Vater's  corpuscles,  69. 

Walrus,  413. 

Veins,  178,  192. 

Wart-hog,  397. 

Velum,  220. 

Weak  fish,  259. 

Vena  cava,  195. 

Weasel,  412. 

Venous  blood,  184. 

Weberian     apparatus,     26, 

Ventral  aorta,  181. 

255- 

Ventral  fins,  231. 

Weevers,  261. 

Ventral  limb,  167. 

Whales,  405. 

Ventral  nerve  roots,  46. 

Whale-bone  whales,  409. 

Ventricles  of  brain,  49. 

White  of  egg,  207. 

Ventricle,  fifth,  57. 

White  matter,  n. 

Ventricle  of  heart,  181. 

White  fish,  256. 

Ventricle  of  larynx,  29. 

White  tissue,  13. 

Vermiform  appendix,  39. 

White  whale,  408. 

Vermis,  54. 

Window  pane,  265. 

Vermilingua,  383. 

Wing  coverts,  330. 

Vertebrae,  135. 

Wirsung's  duct,  40. 

Vertebral  artery,  188. 

Wish-bone,  336. 

Vertebral  bow,  137. 

Wolf,  412. 

Vertebral  column,  134. 

Wolffian  body,  116. 

Vertebrarterial  canal,  145. 

Wolffian  duct,  119,  126. 

Vertebrata,  218. 

Wolf  fish,  261. 

Vertebrates,  origin  of,  215. 

Wolverine,  412. 

Vesicles  of  Savi,  68. 

Woodchuck,  390. 

Vesperugo,  387. 

Woodpecker,  351. 

Vespertilio,  387. 

Wrasse,  260. 

Vespertilionidae,  387. 

Wryneck,  351. 

Vibrissae,  69,  98. 

Vicuna,  398. 

Villi,  38. 

Xenarchi,  259. 

Viper,  325. 

Xenarthra,  382. 

Vipera,  325. 

Xenopterygii,  260. 

Viperidae,  325. 

Xenurus,  383. 

Visceral  clefts,  22. 

Xenopus,  286. 

Visceral  nerves,  64. 

Xerobates,  311. 

Visceral  skeleton,  150,  154. 

Xiphactinus,  256. 

Visual  organs,  78. 

Xiphias,  263. 

Vitelline  membrane,  207. 

Xiphiidae,  263. 

Vitelline  veins,  192. 

Xiphisternum,  148. 

Vitreous  humor,  79. 

Xiphodon,  398. 

Vivera,  412. 

Xiphiplastron,  308. 

INDEX. 


439 


Yak,  400. 
Yellow  tissue,  14. 
Yolk,  206. 
Yolk  stalk,  236. 


Zamicrus,  383. 
Zapus,  390. 


Zebra,  395. 
Zeidae,  261. 
Zeuglodon,  408. 
Ziphius,  409. 
Zoarces,  261. 
Zoarcidae,  261. 
Zona  pellucida,  207. 
Zona  radiata,  207. 


Zonary  placenta,  373. 
Zonula  Zinnii,  83. 
Zonuridae,  320. 
Zygaenidae,  239. 
Zygantrum,  141. 
Zygapophysis,  140. 
Zygomatic  process,  357. 
Zygosphene, 140. 


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